When I started a martial arts campaign with my friends, I decided on a multiversal kumite -- a competition between martial artists drawn from other universes to see who is the absolute "best," who would than face off against a great force of destruction that threatens the multiverse. Of course, that's just the backstory to allow players to make any genre of character they want.
But it also established the boundaries. Players were pivotal supposed to make "martial artists" -- there's a lot of leeway there to make a variety of character types, but the outliers ("purely academic wizard", "a lover not a fighter", etc.) were out. Also, we agreed on certain disadvantages being absent -- you couldn't be Hunted by somebody located in another universe unless they could do planar travel as well, for example.