Hunter - Run very Quickly and do Lot Damage but it have low HP (50 - 75 HP)
Shape-Shifter (I Dont know if its Possible or Not) - who he kills, he can transform himself into it, and the human team cant Know that he is a zombie, so he can Lie to a human and when the Human is Distracted he kill him! (He dont Have inteligence to Use weapons.)
Chamaleon (I Dont know if its Possible or Not) - Chamaleons can be Invisible for 1 Minute, If he attack a human when invisible or the Time from Invisibility is Over he appear again and he can be Invisible just after 5 Minutes or after he die.
And Human Classes:
Enginner: He can Fix cars (Very Usefull) and have a Shotgun.
Camper: He dont appear on radar for zombies, just for humans, he have a sniper rifle and Only one Grenade.
Super-Soldiers - They have 250 HP and Have a MP5 and a Shotgun, They have a Very Strong Punch but they Run Slowly than all Another Classes.
Making that an class that your only Option if you find a zombie and you without a vehicle is Fight the Zombie to the Death.
Well, is that it, hope like my idea, i am waiting a reply!