Seeing as how they're already including Mega Mushrooms in this game, I see no reason why they shouldn't include a Nintendo character or two. I'd like to see Hitmonchan and Hitmonlee as a tag team.
I'd say Hitmonlee and Charizard would be better.
I can't think of how a boxer in Tekken could possibly be different than Steve Fox (Can't let you do that, Steve Fox).
Hitmonlee should be focused entirely on his kicks, sorta like how Steve sways when you press the kick buttons. When pressing the punch buttons, Hitmonlee would do a leg check (it's a muay thai technique, allows you to parry kicks aimed at the legs).
Charizard would have slow attacks, but would specialize in grappling. Many of his grapples would involve flying (flying up and pile driving the opponent for example), biting, and the usage of fire breath. He would also use his wings and tail in combos, and would utilize fire in certain attacks.
If you have accepted Kenshiro as your lord and savior, and are 100% proud of it, you are already dead.