I played a game with once where you could upgrade weapons and armor (star levels) using the same piece of gear with the same star level.

Maybe add that as a way to evolve characters....

Example would be: say a 3* character, Max level, wanted to evolve to 4*, one of the ways to do this would be to use the same character, also 3* star, Max level...

Would make the grind to evolve characters a bit more interesting....