Another suggestion, Star shards.
I don't know if it has been suggest, but, I tought about it.
An event quest where we can collect Stars shards. When you get enough shards, you open a crystal that would give you a star token of random class, and a rare generic.
1 000 for 1*
2 000 for 2*
5 000 for 3*
10 000 for 4*
20 000 for 5*
You can add a star to the champ of that class. Eg. You get a mutant star 4*, you chose your champ and graduate him to 5*. The bad side of that is, he will be 5*, rank/level 1, unawaken.