LoL’s success is because it’s a casual game.

It’s not about making the first one more fair, you aren’t going to have 100 characters equally valuable on a single map while making those characters unique.

If you have 10 maps though you can make those 100 characters equally valuable as a combined average across those 10 maps.

Though that matters more in a competitive game than a casual one. Actually LoL’s lack of skill based competitive nature is likely why they don’t do that. Someone who plays once s month will have an easier time learning the game if it’s a new character that does the same as everyone else than if there’s a new map that makes the characters have to play completely different.