I like the idea of a permanent for Other that, while in play, grants every friendly creature +1|+0 for every mark of its own element that is in play (including opponent's mark.)
Example:
If you have a mark of water, and your opponent has a mark of earth,
all of your earth creatures, and water creatures would gain +1|+0.
If you both had water marks, all your water creatures would gain +2|+0.
I am not sure if this permanent would stack.
Alternatively, one that is much cheaper but buffs both sides, kinda like an eclipse that is mark dependent.
OldTrees, I summon you for balancing input.