Mugen is a crazy thing allowing people to pretty much make who ever they come true in a fighting game. With that said its literally possible for anyone you like to be in Mugen so who did you want first in Mugen? For me I first found out about it through Deku fights against other anime characters so thats kinda what introduced me into it and Deku is the character I want to play the most. The really cool stuff people have been able to do with him like adding many of his moves that he gets later in the manga ( trying to be spoiler free as possible ) really goes to show the cool stuff you can do with Mugen.