Why not adding the Superheros to the game? Like Cartman The Coon, Mysterion, etc, and being Butters Professor Chaos the main boss, legendary card of the theme.
The theme can be good weakening the oponent, like: slows, atk down, prevent status changes and charging, or having really weird effects, like Mysterion can revive himself once he is dead, or The Coon giving speed to allies when charged, the Butters card can do the same as Shaman Token, and also prevent charging and buffs on enemies once he's dead.
Since the colour of the actual themes are the main colour of the main boss, the cards colour can be like cyan or turquoise, like Butters.

Any other good ideas?

Pls Ubisoft, make it happen.

PD: Srry for my bad english, i'm argentinian.