Instead of creating force dual decks, you could do this:
- Have the card have Primary (attack) and Secondary (hp) components liek you currently have
- Have the primary element be your current mark. If primary mark = secondary mark, there effect is voided.
- Have the secondary element be defined.
That way the set could have 12 cards - one for each element, and it would make them all even and able to be played as any dual, instead of forcing particular elements into pairs.
I think adding devour as well probably makes things a bit too OP
description could be:
":mark /
creatures gain +1/0 when :mark creature played and 0/+1 when
creature played"
":mark /
creatures gain +1/0 when :mark creature played and 0/+1 when
creature played"
":mark /
creatures gain +1/0 when :mark creature played and 0/+1 when
creature played"
etc