I would say a lot more variation. Like each character punches their own way instead of everyone looking the same.
Fighting style selection⦠got to have that. Def Jam: Fight for New York had me obsessed with the different fighting style combinations, too bad it was not more in-depth. Maybe a system where you can choose out of a number of moves your character can perform. Such as you can choose how your character punches, etcetera.
Training. Like Fight Night Round 2 had when you could build up your character through training. Not too many games have that, though there are those which you build up your character through fights.
Realistic fighting. Injuries like broken ankles, fatigue, much more weapons (anything you can find), etcetera.
Mostly I am referring to Street Fight games since those always caught my eye.
Online play is a big must so that people all over the world can compete.
SOUTH PAWS!!! I have no seen too many of them in a while except in Boxing games.
A good storyline is another must. Def Jam: Fight for New York had an interesting storyline though, it was too prolonged which made it become boring over time.
Able to buy clothes, accessories, training equipment, etcetera.