a problem is that the foil card is actually a different card;
one can actually include up to 12 chimera's in 1 deck.
While I haven't seen many decks using over 6 chimera's,
I doubt that would hold for, say, a lava golem.
Making a foil card for the cards that you currently can have more than 6 of in 1 deck - pillars and pendulums,
also effects the game: it makes earthquake weaker.
The only card suitable for this, would be one that you don't want more than 6 of anyway.
Very few cards come to mind.
SoPa <- SoI -based deck may use that.
most of the shields could actually work, unless a card that allows you to do things with multiple shields makes it into the game.