Scale up the heads slightly.
More over the top effects for collision. (Possibly lightning)
Logo not terrible and more spiky/angry/brootal.
Color tinting to match the backgrounds.
Interaction between player and the scoreboard (show points scored next to player).
Objects on the character to indicate that they can do damage or not (Armor/skulls above their heads/auras(auras are a terrible idea))
Size is easier to read than color.