I think that you should be able to grow levels in zombies. For example, you get 5exp for every zombies you kill. IT DOES NOT COUNT TOWARDS MULTIPLAYER. When you grow a level you get a point, and you get to spend those points towards classes like, "Tank" everytime you spend a point on the class "tank' it gets harder for zombies to kill you. "Moneybags" you get lots more money per zombie you kill then all the other players, and you are able to donate money to other players.
"Lethal" you do lots more damage to each zombies than all the other players do. So you will be an appreciated player if you have high levels in these classes. "Medic" able to drag teamates to safe places, can revive teamates really fast, and can revive themselves only 3 times per round.Please tell me what you think. Thank you.