Nice OP, game was on my radar and I think I'll bite.
But one question:
What is the difference between a legendary hero and a rare hero? Is it purely stats, like is the max stats a hero can get higher (like in a Gacha game) or do they also have access to more/other powerful abilities?
From my understanding, every hero will likely have different abilities from the other heroes in its same class. The stars/quality of a hero dictate how high it can level.