It's possible to reward players with things that don't necessarily make the game easier but still make the player feel "rewarded." For example, if you were to have a special tournament whose chief reward for being the highest rank was to be able to create a new mutant (or at least provide the idea for one), you make the player feel like he or she is more "part of the world," and more invested in it and less likely to leave. You could have winning fights under certain conditions unlock new game music--after all, after you've heard the original enough, it just gets boring. "Easter eggs" like that add room to explore, and people love the feeling of finding something before anyone else.

A little creativity can go a long way without adding significant game balance issues.