Unsure if this has been suggested before, but what if getting enough of a max level card allowed you to go one step further and evolve it? Changing its properties slightly and being stronger. (i also dont have all the cards yet so if any of the evo abilities overlap ignore it pls)
Things like:
Ice Block+
- Breaking out of the ice releases a snowy wave that slows down foes that get too close for a couple seconds
Cat-turret+
- Cat-Turret now has a second head, for double the fire power!
Tin-man+
- Reinforced armor that deflects low calliber bullets! (reflect bullets from boira)
Akimbo+
- A third gun— but who's pulling the trigger?
Medkit+
- Now also grants an additional bit of health
Predator Vision+
- Reveals your opponent's healthbar as well
Vampire Bullets+
- Now deal minor bleed damage
