I think this idea should only be for single player, because it would kind of be unfair in PvP
Anyways, my idea is that when you get the ability to upgrade your cards, you can also upgrade your mark, but it should be expensive. (30k for one?)
So when you upgrade, you basically get to choose which element you want to add, and can add up to 2 more.
That means that when you upgrade it two time, you can choose +3, +2/+1, or 1+/+1/+1 for your mark.
I think it would help with fighting gods