Any Contraption idea has to be weighed by how it interacts with Steamflogger Boss. Otherwise what is the point? This requires you to have five other cards --a Rigger, two Equipment, and two Vehicles-- before Steamflogger Boss's ability can become relevant. And if you have a second Rigger, then you need at least two more Vehicles that you could use on defense, and two more Equipment if you want to attack with all four Vehicles.
The problem is there is no exact precedent for it. Elemental Resonance is producing mana that is equal to another amount of mana. Nykthos is producing an amount of mana equal to a nonmana number. Nykthos is probably a bit more similar to this. The distinction, to me at least, looks like the difference between mana cost (an amount of mana) and converted mana cost (a number).
There is also tye recent Confiscation Coup which uses "pay an amount of E equal to" to convert a number (the converted mana cost of the targeted creature) into an energy cost.
Seems okay to me for reminder text. I think the actual rules might only need to be just a little longer "Pay an amount of mana equal to this creature's toughness".
The Eternal Command ability word seems pretty easy to accidentally make way too good.
If you want Mazes to colorfix for commander, then you should do so in a way that uses as few colored mana symbols as possible so that they will not be restricted by color identity. Either producing "mana of any color" or functioning like a fetchland are the two obvious paths to take.
Cheap Fetch
Land
~ enters the battlefield tapped.
When ~ enters the battlefield you may search your library for a basic plains or basic forest card, reveal it, and put it in your hand. If you do, shuffle your library afterwards. T: Add W or G to your mana pool.
The last time I saw someone post a dual land that tutored for another land, I iterated on the idea several times before eventually settling on this:
Meandering Stream Land (R) T, Pay 1 life: Add W or U to your mana pool. WU, T, Sacrifice CARDNAME: Search your library for a Plains card and an Island card, put them onto the field tapped, then shuffle your library.
It is a dual land that doubles as a Krosan Verge variant. The mixes of drawbacks are carefully thought out. It costs you life for early color fixing. When your colors are stabilized and you have some spare mana at the end of an opponent's turn, then you can get rid of the life payment and get an additional land.
I later iterated this design one more time with a simpler monocolor version:
Wild Heath Land (U) T, Pay 1 life: Add W to your mana pool. 1W, T, Sacrifice CARDNAME: Search your library for up to two Plains cards, put them onto the field tapped, then shuffle your library.
Self Fetch
Land - Plains
~ enters the battlefield tapped.
When ~ enters the battlefield you may search your library for a plains card, reveal it, and put it in your hand. If you do, shuffle your library afterwards.
This kind of card shouldn't be able to fetch itself, and putting the tutored card into your hand is too good if there is no additional cost. My two variations of this idea, either fetchable or able to fetch duals, but not both:
Plains Trail Land - Plains (C)
CARDNAME enters the field tapped. (T: Add W to your mana pool.)
When Plains Trail enters the field, you may search your library for a basic land card, reveal it, then shuffle your library and put that card on top of it.
Expansive Prairie Land (U)
CARDNAME enters the field tapped. T: Add W to your mana pool.
When CARDNAME enters the field, you may search your library for a Plains card, reveal it, then shuffle your library and put that card on top of it.
It should use "that player" rather than "owner" similar to Head Games.
Search target player’s library for a card and reveal it. That player puts that card into his or her hand and shuffles his or her library. ~ deals damage to that player equal to that card's converted mana cost.
Blade of Souls
I misremembered Banshee's Blade as having that trigger. Maybe I was mixing it up with Blade of the Bloodchief also. It is reasonably different from Blade of the Bloodchief because the counters are on the Equipment and it doesn't have the Vampire interaction.
One concern might be that it can gain counters even if it is not equipped allowing it to work for creature-light control decks. If you want to prevent that, one way to reword it could be to give the ability to the equipped creature:
Equipped creature has "Whenever a creature dies, put a soul counter on Blade of Souls."
Kulphera, Battleborn
If the randomness is meant to prevent abuse, maybe like unearth it should also exile it if it would leave the battlefield. Or like more recent cards, exile the card from the graveyard and create a token that is a copy of it.
Also, nit picky maybe, should this be targeted? After a quick search, I think Grip of Chaos, Goblin Test Pilot, and Scab-Clan Giant are the only cards that involve random targets. That is the point of Grip of Chaos, where as Goblin Test Pilot and Scab-Clan Giant both affect permanents so the opponent should be able interact before the ability resolves.
Abilities that interact randomly with cards in graveyards though do not target the cards. The choice is just made as the ability resolves. Tariel, Reckoner of Souls, Kheru Lich Lord, Charmbreaker Devils, Deadbridge Chant, Fossil Find, Ghoulraiser, etc.
I would have introduced colorless mana sooner and tried to make cards with devoid have generic hybrid {2/W} or colorless hybrid {C/W} mana costs, or at the very least have abilities that cost or produce colorless mana.
I know this is for a Custom Card, but it is noteworthy that Reminder Text doesn't really do anything here as Reminder Text is supposed to remind the player what the rules say and the rules don't support a copy of a permanent spell entering the battlefield. Instead, you either need to update the CR to allow for this or just word your ability a little differently to allow for it. [...]
I think all custom cards that copy permanent spells automatically presume a custom CR change that supports the copy becoming a token as it resolves.
Well, evoke could still do the effect twice like this, but it just take more text space.
Living Knowledge6U Creature - Elemental
Evoke 3U(You may cast this spell for its evoke cost. If you do, it's sacrificed when it enters the battlefield.)
When CARDNAME enters the battlefield, draw two cards, then put the top two cards of your library into your graveyard.
When CARDNAME deals combat damage to a player, you may sacrifice it. If you do, draw two cards, then put the top two cards of your library into your graveyard.
3/3
Anyway, the reminder text might be a little ambiguous to a lot of players. The use of "whenever" makes it sound like you can cast the spell multiple times this way, thus after casting it this way, it must become a creature again. The alternate version might be a little better because of this. Maybe?
Incarnate 6U (You may cast this spell for its incarnate cost. If you do, it becomes an Elemental creature as it resolves. When the creature deals combat damage to a player, you may recast it as a sorcery without paying its mana cost.)
1B Instant
Kicker 1W
Target creature gets +3/-3 until end of turn. If CARDNAME was kicked, you gain life equal to that creature's power.
Strength of Night 1G
Instant
Kicker 1B
Creatures you control get +1/+1 until end of turn. If CARDNAME was kicked, Zombie creatures you control get an additional +2/+2 until end of turn.
Vigorous Charge G
Instant
Kicker W
Target creature gets +1/+1 and gains trample until end of turn. If CARDNAME was kicked, untap that creature and it gains lifelink until end of turn.
There is also tye recent Confiscation Coup which uses "pay an amount of E equal to" to convert a number (the converted mana cost of the targeted creature) into an energy cost.
If you want Mazes to colorfix for commander, then you should do so in a way that uses as few colored mana symbols as possible so that they will not be restricted by color identity. Either producing "mana of any color" or functioning like a fetchland are the two obvious paths to take.
Land (R)
T, Pay 1 life: Add W or U to your mana pool.
WU, T, Sacrifice CARDNAME: Search your library for a Plains card and an Island card, put them onto the field tapped, then shuffle your library.
It is a dual land that doubles as a Krosan Verge variant. The mixes of drawbacks are carefully thought out. It costs you life for early color fixing. When your colors are stabilized and you have some spare mana at the end of an opponent's turn, then you can get rid of the life payment and get an additional land.
I later iterated this design one more time with a simpler monocolor version:
Land (U)
T, Pay 1 life: Add W to your mana pool.
1W, T, Sacrifice CARDNAME: Search your library for up to two Plains cards, put them onto the field tapped, then shuffle your library.
This kind of card shouldn't be able to fetch itself, and putting the tutored card into your hand is too good if there is no additional cost. My two variations of this idea, either fetchable or able to fetch duals, but not both:
Land - Plains (C)
CARDNAME enters the field tapped.
(T: Add W to your mana pool.)
When Plains Trail enters the field, you may search your library for a basic land card, reveal it, then shuffle your library and put that card on top of it.
Land (U)
CARDNAME enters the field tapped.
T: Add W to your mana pool.
When CARDNAME enters the field, you may search your library for a Plains card, reveal it, then shuffle your library and put that card on top of it.
Search target player’s library for a card and reveal it. That player puts that card into his or her hand and shuffles his or her library. ~ deals damage to that player equal to that card's converted mana cost.
Is Earthen Debris too complicated for common? Have there been other Walking Wall effects at common recently?
I misremembered Banshee's Blade as having that trigger. Maybe I was mixing it up with Blade of the Bloodchief also. It is reasonably different from Blade of the Bloodchief because the counters are on the Equipment and it doesn't have the Vampire interaction.
One concern might be that it can gain counters even if it is not equipped allowing it to work for creature-light control decks. If you want to prevent that, one way to reword it could be to give the ability to the equipped creature:
Kulphera, Battleborn
If the randomness is meant to prevent abuse, maybe like unearth it should also exile it if it would leave the battlefield. Or like more recent cards, exile the card from the graveyard and create a token that is a copy of it.
Also, nit picky maybe, should this be targeted? After a quick search, I think Grip of Chaos, Goblin Test Pilot, and Scab-Clan Giant are the only cards that involve random targets. That is the point of Grip of Chaos, where as Goblin Test Pilot and Scab-Clan Giant both affect permanents so the opponent should be able interact before the ability resolves.
Abilities that interact randomly with cards in graveyards though do not target the cards. The choice is just made as the ability resolves. Tariel, Reckoner of Souls, Kheru Lich Lord, Charmbreaker Devils, Deadbridge Chant, Fossil Find, Ghoulraiser, etc.
I think all custom cards that copy permanent spells automatically presume a custom CR change that supports the copy becoming a token as it resolves.
Living Knowledge 6U
Creature - Elemental
Evoke 3U (You may cast this spell for its evoke cost. If you do, it's sacrificed when it enters the battlefield.)
When CARDNAME enters the battlefield, draw two cards, then put the top two cards of your library into your graveyard.
When CARDNAME deals combat damage to a player, you may sacrifice it. If you do, draw two cards, then put the top two cards of your library into your graveyard.
3/3
Anyway, the reminder text might be a little ambiguous to a lot of players. The use of "whenever" makes it sound like you can cast the spell multiple times this way, thus after casting it this way, it must become a creature again. The alternate version might be a little better because of this. Maybe?
Incarnate 6U (You may cast this spell for its incarnate cost. If you do, it becomes an Elemental creature as it resolves. When the creature deals combat damage to a player, you may recast it as a sorcery without paying its mana cost.)
If you want it to have a blue color identity for Commander's sake, then you can just make it produce blue mana like Tolarian Academy.
CARDNAME can block any number of creatures and blocks each attacking creature if able.
Instant
Kicker 1W
Target creature gets +3/-3 until end of turn. If CARDNAME was kicked, you gain life equal to that creature's power.
Strength of Night 1G
Instant
Kicker 1B
Creatures you control get +1/+1 until end of turn. If CARDNAME was kicked, Zombie creatures you control get an additional +2/+2 until end of turn.
Vigorous Charge G
Instant
Kicker W
Target creature gets +1/+1 and gains trample until end of turn. If CARDNAME was kicked, untap that creature and it gains lifelink until end of turn.