Well probably see a bunch of <> in mirrodin 3 so they can push artifacts hard so they won't suck again but won't go all mirrodin 1. Of course they could make it a subtheme for any block they want as long as they have a colorless faction.
I'm not sure that <> fits for artifacts. At least, <> in artifact costs is at the same level as having artifacts with colored mana costs. Meaning it's something that requires justification. Colored artifacts have vital components composed of both metal and flesh, so then what is a <> artifact made of when <> is associated largely with the chaotic and immaterial?
We know Wastes and <> cost cards neatly fit into any land-themed block because of the presence of utility lands, but is there any other theme they can go along with? If not, then there's a problem, because it would mean Wastes can only come back during land blocks, which would make them all similar to zendikar. Unless, like I said, there's another way to support them in-block than with a land theme.
Sorry if this has already been mentioned, but another reason that it can't be a type of hybrid mana is because the mana symbol is normal sized. Every type of hybrid mana symbol (two color, 2brid, and phyrexian) is enclosed within a slightly larger than normal circle.
We already have a common and a mythic, so here's two for the other rarities:
Shadowborn Feast6B
Instant (U)
~ costs 1 less to cast for each creature card in your graveyard.
~ deals 4 damage to target creature or player and you gain 4 life. If you control a Demon, draw a card.
Shadowborn Vessel1BB
Creature - Human Cleric (R)
Flying
As long as there are six or more creature cards in your graveyard, ~ gets +2/+3 and is a Demon in addition to its other creature types.
Demon creatures you control have haste.
3/3
I had a five player game where I was using Daretti, Scrap Savant and I won the game with exactly 1 life and 0 cards in my library(I was getting attacked by a creature wielding Sword of Body and Mind for part of the game, my graveyard also got exiled once with Relic of Progenitus). I went a full turn cycle at 1 life without anybody committing to kill me. On my turn, I cast All is Dust and Pyrite Spellbomb, while I had two Salvaging Stations(one was Sculpting Steel). There were a couple artifact creatures left on the board, but the spellbombs + station would kill any that came at me. On my next turn, I assembled Basalt Monolith + Rings of Brighthearth combo and used the mana to destroy all of my opponents' permanents with Spine of Ish Sah. In response to this combo, however, Melek, Izzet Paragon cast Mystical Tutor for Reforge the Soul and miracle'd it with the draw from Sensei's Divining Top. I had exactly 7 cards in my library when he did that. I was able to recur Scroll Rack with Codex Shredder though, and win next turn with infinite tokens generated off of Summoning Station using infinite untaps from brightheart'd Voltaic Key. I was drawing a lot of cards off of Salvaging Station + Phyrexian Furnace earlier in that game, so I cut it unimaginably close that game.
Walking SigilsU
Creature - Illusion (U)
Graft 1
When ~ dies, look at the top X cards of your library, where X is the number of +1/+1 counters among creatures you control. Put one of them into your hand and the rest on the bottom of your library in any order.
0/0
Verakis Chargers2R
Creature - Human Warrior (U)
Graft 2
Whenever a +1/+1 counter is removed from ~, creatures you control get +1/+0 and gain haste until end of turn.
0/0
Ash Spirit1WB
Creature - Spirit (R)
Graft 2
When ~ dies, if it had no +1/+1 counters on it, destroy target permanent.
0/0
Forge Echoes2R
Enchantment (R)
Whenever a nontoken artifact enters the battlefield under your control, you may pay R. If you do, put a token onto the battlefield that is a copy of that artifact. That token gains haste. Exile it at the beginning of the next end step.
Crooked Clock4
Artifact (R)
Whenever a permanent you control becomes untapped, untap it.
Melting Station4
Artifact (U) T, Sacrifice a land: ~ deals 2 damage to each opponent.
Whenever a land enters the battlefield, you may untap ~.
Printing Station7
Artifact (R) T: Draw a card.
Whenever a land is put into a graveyard from the battlefield, you may untap ~.
Reactor Golem7
Artifact Creature - Golem (R)
Whenever ~ or another artifact creature dies, untap up to two target artifacts.
6/8
Scouring Station3
Artifact (U) T, Discard a card: Target player puts the top two cards of his or her library into his or her graveyard.
Whenever a player draws a card, you may untap ~.
Tilling Station6
Artifact (R) T: Return target land card from your graveyard to the battlefield.
Whenever a player discards a card, you may untap ~.
I made cards in the past that were similar to some of these.
My version of Mischief Monger only affected instants/sorceries. My version of Nosedive made the creature lose flying until end of turn. I had a drawing enchantment that used the same cost as Nolven Underground, except it also made you lose life with every activation.
Edit: Also, my version of Nosedive just hit all creatures without flying.
The friendly fighters and Void's Pull are a bit weak. The Crier is more of a red effect because it's burst mana. I was about to say Static Cloud was too inconsistent in effect but then I noticed it didn't say "or ability". Cards that amplify the power of burn have historically never been that good, so I'd be hesitant to ever call something like this too strong.
It's not worth it if you aren't going to use both modes. It doesn't suddenly become okay to overpay for card draw just because you're playing mono-red.
I really like this card and have fond memories of playing in a bygone era when enchantments didn't have to be creatures to be relevant. I'd love for this to be a thing, but in the current state of Magic, cards that don't have an immediate impact have an uphill battle to work their way into constructed decklists. Maybe this is cheap enough and the effect strong enough to do the trick, but color me skeptical at the moment.
But in this case, I really wish it was a creature so it wasn't dead in multiples.
2. The card has to cost less than a card that would represent its weaker mode(so this must be at least less than 1RR).
I'm not sure that <> fits for artifacts. At least, <> in artifact costs is at the same level as having artifacts with colored mana costs. Meaning it's something that requires justification. Colored artifacts have vital components composed of both metal and flesh, so then what is a <> artifact made of when <> is associated largely with the chaotic and immaterial?
Shadowborn Feast 6B
Instant (U)
~ costs 1 less to cast for each creature card in your graveyard.
~ deals 4 damage to target creature or player and you gain 4 life. If you control a Demon, draw a card.
Shadowborn Vessel 1BB
Creature - Human Cleric (R)
Flying
As long as there are six or more creature cards in your graveyard, ~ gets +2/+3 and is a Demon in addition to its other creature types.
Demon creatures you control have haste.
3/3
Walking Sigils U
Creature - Illusion (U)
Graft 1
When ~ dies, look at the top X cards of your library, where X is the number of +1/+1 counters among creatures you control. Put one of them into your hand and the rest on the bottom of your library in any order.
0/0
Verakis Chargers 2R
Creature - Human Warrior (U)
Graft 2
Whenever a +1/+1 counter is removed from ~, creatures you control get +1/+0 and gain haste until end of turn.
0/0
Ash Spirit 1WB
Creature - Spirit (R)
Graft 2
When ~ dies, if it had no +1/+1 counters on it, destroy target permanent.
0/0
Enchantment (R)
Whenever a nontoken artifact enters the battlefield under your control, you may pay R. If you do, put a token onto the battlefield that is a copy of that artifact. That token gains haste. Exile it at the beginning of the next end step.
Crooked Clock 4
Artifact (R)
Whenever a permanent you control becomes untapped, untap it.
Melting Station 4
Artifact (U)
T, Sacrifice a land: ~ deals 2 damage to each opponent.
Whenever a land enters the battlefield, you may untap ~.
Printing Station 7
Artifact (R)
T: Draw a card.
Whenever a land is put into a graveyard from the battlefield, you may untap ~.
Reactor Golem 7
Artifact Creature - Golem (R)
Whenever ~ or another artifact creature dies, untap up to two target artifacts.
6/8
Scouring Station 3
Artifact (U)
T, Discard a card: Target player puts the top two cards of his or her library into his or her graveyard.
Whenever a player draws a card, you may untap ~.
Tilling Station 6
Artifact (R)
T: Return target land card from your graveyard to the battlefield.
Whenever a player discards a card, you may untap ~.
My version of Mischief Monger only affected instants/sorceries. My version of Nosedive made the creature lose flying until end of turn. I had a drawing enchantment that used the same cost as Nolven Underground, except it also made you lose life with every activation.
Edit: Also, my version of Nosedive just hit all creatures without flying.
The friendly fighters and Void's Pull are a bit weak. The Crier is more of a red effect because it's burst mana. I was about to say Static Cloud was too inconsistent in effect but then I noticed it didn't say "or ability". Cards that amplify the power of burn have historically never been that good, so I'd be hesitant to ever call something like this too strong.
But in this case, I really wish it was a creature so it wasn't dead in multiples.