Here are two version of the same concept, which use slightly different mechanics:
Dust Transmuter2
Artifact (M) X, T: Exile a card from your hand with converted mana cost X. T: Cast a card from your hand with converted mana cost equal to a card exiled with CARD.
Dust Transmuter v2X
Artifact (M) Imprint - When CARD enters the battlefield, exile a card from your hand with converted mana cost X. It is imprinted on CARD. T: Cast a card from hand with the same converted mana cost as the imprinted card.
Flashback meets madness:
Sudden Craze1R
Sorcery(U)
Add RRR to your mana pool. If you cast CARD with its madness cost, instead you may cast a card from your graveyard with converted mana cost X. Exile both the cast card and CARD as the cast card resolves.
Madness XR
For a Commander set:
Pillar of Sight3
Artifact (U) T, Choose an opponent: Add 1 to your mana pool. You may look at a random card from the chosen opponent's hand.
Your opponents play with the top card of their library revealed.
I have a confession to make: I really like Hearthstone. I picked the game up a few weeks ago and I'm hooked. It's everything digital Magic should be, but with some new and unique mechanics which keep things fresh. Some of these mechanics would be interesting if brought into Magic, so here goes:
Inspire (Whenever you activate an activated ability of your commander, [effect])
A niche but interesting mechanic which would find a home in a commander set.
Banana King3GG
Creature - Ape (R) Inspire - Whenever you activate an activated ability of your commander, put a +1/+1 counter on each creature you control.
4/4
Overload N (As an additional cost to cast this spell, choose N tapped lands you control. Those lands don't untap during your next untap step.)
An all in mechanic which makes for interesting mana management scenarios.
Void BlastR
Instant (C)
Overload 1 (As an additional cost to cast this spell, choose one tapped land you control. That land doesn't untap during your next untap step.
CARD deals 3 damage to target creature or player.
Rage (When this creature attacks, you may instead remove it from combat and have it fight target creature.)
Attacking creatures is something which makes Hearthstone play well, and is something that I think could find a home in Magic.
Frenzy Plant2G
Creature - Plant (C)
Rage
3/1
Lure (This creature must be blocked if able.)
Keywording an existing ability, as it fits the Taunt ability from Hearthstone so well.
Banjo Bear2G
Creature - Bear (U)
Trample
Lure
3/3
Spell Damage N (Whenever an instant or sorcery you control would deal damage, instead it deals that much damage +N)
A limited format defined by slinging large amounts of damage around would be quite exciting.
Ullon Magi2U
Creature - Orc Wizard (C)
Spell Damage +1 (Whenever an instant or sorcery you control would deal damage, instead it deals that much damage +1)
1/4
You get N armor counters. (Whenever you would take damage, you may instead remove that many armor counters.)
Playing around in the "counters on players" space, I think with the technology of counter cards (like poison counters) armor could be viable.
Divine Shield2W
Sorcery (U)
You get five armor counters. (Whenever you would take damage, you may instead remove that many armor counters.)
Draw a card.
Burrow (You may put this card from your hand face down into exile for 2.)
AND
[trigger], play this creature from exile.
Basically a way to create trap creatures. This also works as a cheap version of Morph, since you can scale the costs down to 2 rather than 3.
Hidden Predator3R
Creature - Horror (C)
Burrow (You may put this card from your hand face down into exile for 2.)
Haste
When an opponent plays an artifact or planeswalker you may turn CARD face up and put it onto the battlefield.
3/1
Extract - [cost] ([cost], Exile this artifact: You get an emblem with [global version of effect]. Activate this ability only when you could cast a sorcery.)
AND
[effect]
Basically you can take a potent artifact (typically an Equipment) and extract it's power so it can be used by your whole team permanently.
Kossith's Blade5
Artifact - Equipment (R)
Equip 3
Equipped creature gets +4/+1 and gains deathtouch
Extract 7 (7, Exile this artifact: You get an emblem with "Creatures you control get +4/+1 and have deathtouch". Activate this ability only when you could cast a sorcery.)
-1 Arbor Elf: With only half a dozen Forests, this just doesn't hit often enough.
-1 Phantasmal Image: The primary utility of this creature was as a cheap way to kill commanders. With that gone it's just another dude. I still need to run the numbers, but it might be worth including if there are enough creatures which having two of on the board is ideal.
-1 Willbender: Stratus Dancer is effectively a direct upgrade. This shares a slot with Coral Trickster, and I prefer the Trickster.
-1 Genesis Hydra: This is a very, very good card in Animar. It does everything that you want at an extremely affordable price. However it is extremely anti-synergistic with Cascade (as are all Hydras) and Cascade is simply better.
-1 Deceiver Exarch: This was originally included as redundancy for Pestermite. However Coral Trickster is just better, and allows me to concentrate slots.
-1 Kruphix, God of Horizons: This is a strong card. However it is suboptimal in Animar and I'd rather just focus on winning.
-1 Artisan of Kozilek: The Eldrazi are just too expensive to justify, even with relevant abilities. It is rare that you can generate enough mana (including Animar's free mana) to reliably hit this and do something else on your turn.
-1 Bane of Progress: This is a surprisingly dead card considering how many essential artifacts there are in the deck.
-1 Fire-Lit Thicket: This is a debatable choice. I need to be running at least 2 Mountains, but I'm still not sure which are the four best dual cycles to run.
-1 Grove of the Burnwillows: It's amazing how much 1 life matters in EDH.
-1 Mosswort Bridge: My cards are already cheap, I'd rather get a bunch of free power.
+1 Stratus Dancer: Direct upgrade to Willbender.
+1 Den Protector: Direct upgrade to Artisan of Kozilek.
+1 Shaman of the Great Hunt: I've had a lot of success with this guy. He makes every single creature on your side of the field into threats extremely quickly, not to mention he can almost always be a free Divination or Harmonize every turn.
+1 Ancestral Statue: I shouldn't have to explain this card.
+1 Lodestone Golem: This is an extremely potent control card which, as a creature, still progresses us towards victory. It is very easy to bounce this guy to cast our spells and keep him on the field on our opponents' turns.
+1 Primordial Sage: Redundancy on Soul of the Harvest. This or Soul plus any of the numerous 0-mana bounce engines in the deck turns into a lot of cards very quickly.
+1 Grave Sifter: This should net you at least two, if not three, cards from your graveyard every time you cast it.
+1 Beastcaller Savant: This turns many of the 1-mana bounce engines into 0-mana engines.
+1 Cinder Glade: This is effectively a dual land which enters the battlefield tapped. There is nothing wrong with that.
+1 Oran-Rief, the Vastwood. This spreads our opponents' kill spells and can translate into a lot of power over the course of the game. It combos beautifully with Shaman of the Great Hunt.
+1 Mountain: All tri-colour EDH decks should have a minimum of 8 basics, with at least 2 of each type.
Frank a good list to look at that includes Craterhoof and plays a more straightforward version of Animar than Gelf's list (which has a lot of tutors and non-creatures relatively speaking) have a look at my list. I've run Survival and Recruiter, both were too good for my meta but if you are going to absolute power are very good options, Recruiter much more so than Survival. Consecrated Sphinx and Craterhoof I run as they are both awesomely powerful cards, the former as a way to quickly get back in the game and the later as one of the best creature based win-cons in EDH. Greater Good is frankly not good enough for Animar. You want to be starting your engine on turn 4, not dropping an enchantment that does nothing (remember turn 3 or 2 should ALWAYS be Animar). I've run Momir-Vig and I can tell you he's very underwhelming. There's a lot of good red creatures for Animar (more in recent years) which means its even harder to activate him consistently than when Animar first came out. Vig just doesn't get there in EDH, as a commander or in the 99.
Ah sorry, I realize I was a bit short. [deck*][/deck*] (remove the asterisks) around your cards will format them into the deck format you see on other people's deck threads. Adding [*deck=blah blah] (again remove the asterisks) will give you "blah blah" as the title of your deck. Lines in the deck tags with numbers will show up as cards, without as headers. I organize my EDH lists by Creatures, then Spells, and finally Lands with Creatures and Spells sorted by cmc. Lands I sort rare dual, tri+, non-rare dual, mono, colourless, basic, keeping cycles togethor.
We've known for a while that the rules commitee doesn't actually care about making EDH a good format. If they did, cards like Gaea's Cradle, Sol Ring, or Defense of the Heart would've been banned a long time ago.
Then they aren't "win more". Investing more resources into a winning strategy than is needed is unnecessary and frivolous. "Win more" is NOT a fallacy, it is the idea that cards whose only purpose is to advance you towards an inevitable victory are unnecessary once that victory has been confirmed inevitable. In the area of design, it would be interesting to discuss what makes a card or mechanic winmore. Does it have to do with efficiency (such as with Sweep)? Does it have to do with timing (such as with most big creatures)? Are there aspects of play other than efficiency or timing which control whether a card is winmore?
In the specific case of Formidable, the mechanic itself is not winmore. The specific cards using Formidable are not winmore. Winmore is a relative condition, determined by the specific interactions between your deck and your opponent's deck. Casting Lightning Bolt on an opponent at 1 while you have Shock in hand is a great example of winmore. It is better to hold the Bolt back because it is a more efficient (and therefore more powerful) card, simply the Shock will suffice. The correct choice in Magic, like the correct choice in any strategic scenario, is always going to be the maximally efficient, minimally powerful option. You could use a howitzer to take out that car, but if you have a bazooka it's just as powerful (the car will still be removed) but far more efficient (a bazooka is far more replacable once it's position is revealed, "spent", than a howitzer). The trick to designing cards that aren't winmore is to design cards (or groups of cards) that are unique in a given environment.
You're not writing for a prompt, so the more words the merrier. If a point is better expressed in a paragraph than a single anecdote (and they always are) then go with the extra material. I personally disgree on the placement of Miracles and Kinship (they both play in the same unknown space and are effectively the same), I do like the concept of the Banding Scale. It would be useful to assign various evergreen and other "basic" mechanics to the Banding Scale (aka the big BS) to give players a better intuitive understanding of what each point represents:
Vigilance (2) - About as simple to understand and play with (or against) as flying (both mehanics add or remove a single blocker/attacker). However the two turn nature or Vigilance (it adds a blocker next turn) leads me to push it up to a 2.
First Strike (3) - Highly grokable flavour means that it is nigh impossible to misinterpret how this mechanic works. However the addition of an extra damage step and the ability to interact inbetween the two damage steps means that playing with (or against) first strike is significantly more complicated.
Lifelink (1) - Lifelink feels good and as such is much easier to understand and play well. Life totals are mostly irrelevant to board state, meaning Lifelink doesn't affect board complexity directly.
Haste (1) - As soon as a player understands that creatures can't attack or tap the turn they enter the battlefield, they understand haste.
Trample (3) - Trample falls into the same category as First Strike, easy to understand but the rules implicatons means there is a lot of hidden complexity.
1
Dust Transmuter 2
Artifact (M)
X, T: Exile a card from your hand with converted mana cost X.
T: Cast a card from your hand with converted mana cost equal to a card exiled with CARD.
Dust Transmuter v2 X
Artifact (M)
Imprint - When CARD enters the battlefield, exile a card from your hand with converted mana cost X. It is imprinted on CARD.
T: Cast a card from hand with the same converted mana cost as the imprinted card.
Flashback meets madness:
Sudden Craze 1R
Sorcery(U)
Add RRR to your mana pool. If you cast CARD with its madness cost, instead you may cast a card from your graveyard with converted mana cost X. Exile both the cast card and CARD as the cast card resolves.
Madness XR
For a Commander set:
Pillar of Sight 3
Artifact (U)
T, Choose an opponent: Add 1 to your mana pool. You may look at a random card from the chosen opponent's hand.
Your opponents play with the top card of their library revealed.
1
Inspire (Whenever you activate an activated ability of your commander, [effect])
A niche but interesting mechanic which would find a home in a commander set.
Banana King 3GG
Creature - Ape (R)
Inspire - Whenever you activate an activated ability of your commander, put a +1/+1 counter on each creature you control.
4/4
Overload N (As an additional cost to cast this spell, choose N tapped lands you control. Those lands don't untap during your next untap step.)
An all in mechanic which makes for interesting mana management scenarios.
Void Blast R
Instant (C)
Overload 1 (As an additional cost to cast this spell, choose one tapped land you control. That land doesn't untap during your next untap step.
CARD deals 3 damage to target creature or player.
Rage (When this creature attacks, you may instead remove it from combat and have it fight target creature.)
Attacking creatures is something which makes Hearthstone play well, and is something that I think could find a home in Magic.
Frenzy Plant 2G
Creature - Plant (C)
Rage
3/1
Lure (This creature must be blocked if able.)
Keywording an existing ability, as it fits the Taunt ability from Hearthstone so well.
Banjo Bear 2G
Creature - Bear (U)
Trample
Lure
3/3
Spell Damage N (Whenever an instant or sorcery you control would deal damage, instead it deals that much damage +N)
A limited format defined by slinging large amounts of damage around would be quite exciting.
Ullon Magi 2U
Creature - Orc Wizard (C)
Spell Damage +1 (Whenever an instant or sorcery you control would deal damage, instead it deals that much damage +1)
1/4
You get N armor counters. (Whenever you would take damage, you may instead remove that many armor counters.)
Playing around in the "counters on players" space, I think with the technology of counter cards (like poison counters) armor could be viable.
Divine Shield 2W
Sorcery (U)
You get five armor counters. (Whenever you would take damage, you may instead remove that many armor counters.)
Draw a card.
Burrow (You may put this card from your hand face down into exile for 2.)
AND
[trigger], play this creature from exile.
Basically a way to create trap creatures. This also works as a cheap version of Morph, since you can scale the costs down to 2 rather than 3.
Hidden Predator 3R
Creature - Horror (C)
Burrow (You may put this card from your hand face down into exile for 2.)
Haste
When an opponent plays an artifact or planeswalker you may turn CARD face up and put it onto the battlefield.
3/1
Extract - [cost] ([cost], Exile this artifact: You get an emblem with [global version of effect]. Activate this ability only when you could cast a sorcery.)
AND
[effect]
Basically you can take a potent artifact (typically an Equipment) and extract it's power so it can be used by your whole team permanently.
Kossith's Blade 5
Artifact - Equipment (R)
Equip 3
Equipped creature gets +4/+1 and gains deathtouch
Extract 7 (7, Exile this artifact: You get an emblem with "Creatures you control get +4/+1 and have deathtouch". Activate this ability only when you could cast a sorcery.)
1
13th of October, 2015
Explanations
-1 Arbor Elf: With only half a dozen Forests, this just doesn't hit often enough.
-1 Phantasmal Image: The primary utility of this creature was as a cheap way to kill commanders. With that gone it's just another dude. I still need to run the numbers, but it might be worth including if there are enough creatures which having two of on the board is ideal.
-1 Willbender: Stratus Dancer is effectively a direct upgrade. This shares a slot with Coral Trickster, and I prefer the Trickster.
-1 Genesis Hydra: This is a very, very good card in Animar. It does everything that you want at an extremely affordable price. However it is extremely anti-synergistic with Cascade (as are all Hydras) and Cascade is simply better.
-1 Deceiver Exarch: This was originally included as redundancy for Pestermite. However Coral Trickster is just better, and allows me to concentrate slots.
-1 Kruphix, God of Horizons: This is a strong card. However it is suboptimal in Animar and I'd rather just focus on winning.
-1 Artisan of Kozilek: The Eldrazi are just too expensive to justify, even with relevant abilities. It is rare that you can generate enough mana (including Animar's free mana) to reliably hit this and do something else on your turn.
-1 Bane of Progress: This is a surprisingly dead card considering how many essential artifacts there are in the deck.
-1 Fire-Lit Thicket: This is a debatable choice. I need to be running at least 2 Mountains, but I'm still not sure which are the four best dual cycles to run.
-1 Grove of the Burnwillows: It's amazing how much 1 life matters in EDH.
-1 Mosswort Bridge: My cards are already cheap, I'd rather get a bunch of free power.
+1 Stratus Dancer: Direct upgrade to Willbender.
+1 Den Protector: Direct upgrade to Artisan of Kozilek.
+1 Shaman of the Great Hunt: I've had a lot of success with this guy. He makes every single creature on your side of the field into threats extremely quickly, not to mention he can almost always be a free Divination or Harmonize every turn.
+1 Ancestral Statue: I shouldn't have to explain this card.
+1 Lodestone Golem: This is an extremely potent control card which, as a creature, still progresses us towards victory. It is very easy to bounce this guy to cast our spells and keep him on the field on our opponents' turns.
+1 Primordial Sage: Redundancy on Soul of the Harvest. This or Soul plus any of the numerous 0-mana bounce engines in the deck turns into a lot of cards very quickly.
+1 Grave Sifter: This should net you at least two, if not three, cards from your graveyard every time you cast it.
+1 Beastcaller Savant: This turns many of the 1-mana bounce engines into 0-mana engines.
+1 Cinder Glade: This is effectively a dual land which enters the battlefield tapped. There is nothing wrong with that.
+1 Oran-Rief, the Vastwood. This spreads our opponents' kill spells and can translate into a lot of power over the course of the game. It combos beautifully with Shaman of the Great Hunt.
+1 Mountain: All tri-colour EDH decks should have a minimum of 8 basics, with at least 2 of each type.
1
1
1
2
1
In the specific case of Formidable, the mechanic itself is not winmore. The specific cards using Formidable are not winmore. Winmore is a relative condition, determined by the specific interactions between your deck and your opponent's deck. Casting Lightning Bolt on an opponent at 1 while you have Shock in hand is a great example of winmore. It is better to hold the Bolt back because it is a more efficient (and therefore more powerful) card, simply the Shock will suffice. The correct choice in Magic, like the correct choice in any strategic scenario, is always going to be the maximally efficient, minimally powerful option. You could use a howitzer to take out that car, but if you have a bazooka it's just as powerful (the car will still be removed) but far more efficient (a bazooka is far more replacable once it's position is revealed, "spent", than a howitzer). The trick to designing cards that aren't winmore is to design cards (or groups of cards) that are unique in a given environment.
1
1
Vigilance (2) - About as simple to understand and play with (or against) as flying (both mehanics add or remove a single blocker/attacker). However the two turn nature or Vigilance (it adds a blocker next turn) leads me to push it up to a 2.
First Strike (3) - Highly grokable flavour means that it is nigh impossible to misinterpret how this mechanic works. However the addition of an extra damage step and the ability to interact inbetween the two damage steps means that playing with (or against) first strike is significantly more complicated.
Lifelink (1) - Lifelink feels good and as such is much easier to understand and play well. Life totals are mostly irrelevant to board state, meaning Lifelink doesn't affect board complexity directly.
Haste (1) - As soon as a player understands that creatures can't attack or tap the turn they enter the battlefield, they understand haste.
Trample (3) - Trample falls into the same category as First Strike, easy to understand but the rules implicatons means there is a lot of hidden complexity.