Going to suggest a kitchen sink kind of thing, either because that's just the way the world is or due to bleedthrough from other realities or... Something.  Makes it easier to allow people to use whatever they want.

...And I think as long as we're not tossing around Silver Age Supermen or Super Saiyans or overdeities or anything ridiculous like that, power levels shouldn't be something we'll need to worry about too much.  As long as everyone is sensible, it should work out fine. XD