I'm going to give his muscles more definition and add abs, and I'm going to put a slit on the sides of his shorts.
A few specifics - mouth guard yes or no? - blue and red or grey and red? do you see a combination you like - I can try anything imaginable, though I think that limiting his clothing to two colors will help readability and unify Joe's character. - you like his spiky hair? I see most MMA fighters have shorter hair, but I kept his hair just like the karate original. - do you like the circular background more than my top-down view of the octagon cage? I can also try a background like he's standing in a cage.