I've played this game several times now, and there always seems to be something missing in the normal gameplay.
Suggestion: Onyx needs a special move like most side-scrolling fighters have to do serious damage and the ability to grab/throw her opponents to break up the repetitive punching/kicking. In this area, as befits the personality of the game,
I suggest that you give Onyx a kind of fun counter to the sudden sexual advance move of her opponents that reverses their move with a move of Onyx's own, like grabbing their dick and holding onto it to assault them while they are surprised and helpless.
This would initiate a different sexual interaction and animation with each type of enemy just like the sexual interactions and animations already in the game that are used on Onyx.
Whenever, an enemy initiates their sudden sexual advance special, they should prepare early at the eye gleam effect by unzipping their pants and getting their cock out. Only one change needs made and I think it is a good one anyway as it is an easy way to make them vulnerable to Onyx.
Importantly, this allows the player of Onyx the ability to turn the tables instead of being the one getting fucked all the time.