Hybrid-Class-Champions
As you see Karnak is the only champion in the game that requires two different classes for rank up. Why don't you make that a Thing. Make him a mixture out of two classes. Same goes for new champs. Some of them can be a hybrid champ having two classes.
Suggestion:
Crystal: Cosmic+ Mystic
Gorgon : Cosmic + Science
Legion: Mutant + Mystic