OK I'll probably get some hate for this but how about a level/perk system for survivors and maybe for special infected in versus? I just think they try to make the characters all totally different in personality and look, yet they all play exactly the same. There is a crazy amount of things you could add that could differentiate the characters:
More melee before fatigue sets in
Not hobbled with low health
Hold more ammo/health items
Akimbo primary weapons
And those are just the basics
Special infected could have more damage, shorter cooldown on special attacks, more health
It would fundamentally change the game but it could be cool.