I feel like the zombies in this game are just kinda bleah. Huge horde chases after you and that’s it. Hop through a window and club em to death one by one. The total lack of horror when dealing with them. They are more of an annoyance then anything.
So here is what I propose.
Roamers/Wanderers/Runners/Chasers - The current regular zombies would be renamed this. They would spawn en-mass inside cities outside of buildings.
Squatter/Brooder/Lurker - The sit around inside buildings not really doing much. Known to sit almost motionless for large lengths of time. Can be hard to spot inside a darkened building (perception check?). Maybe use all three names for a minor variety of the same general type of zombie. Spawns in houses.
Dragger/Grasper/Stubby - A zombie that can’t really walk anymore. Can be hard to see in darkened allies and houses (perception check?). Known to grab onto humans as they run by. Temporarily trapping them until dealt with. Melee weapons are particularly effective vs these zombies. If a player keeps trying to move out of the square next to the zombie they can make an attempt to break free of the grasp. Not very danger in and of themselves. Usually zombie is very weak. Very low to no movement rate. Very minor bites.
Customer - Basically just a roamer that spawns in commercial buildings. Garage, Restaurant, Any of the actual stores, etc. Just not in houses. Maybe just spawn some roamers inside the buildings instead. shrugs
Employee - Basically just another roamer, but could be used to drop keys and what not to locked containers inside a building.
Wild One/Wildling/Tarzan/Ape - A zombie wandering the wilderness, usually their clothes are considerably tattered.
Ghouls - Highly irradiated zombies. Think like the ghouls on fallout. Scarecrow like. Glowing ones could even be high end ghouls like in Fallout. 
Now making these guys mobs and then just spawning them in is one way to do it. I, however, have another method in mind. Think of it like a race/class combination. A Roamer Customer. A Police Dragger. A Mechanic Lurker. A Chaser Bartender. Some don’t work well together. Like a lurking Wildlign.
Also, give them a range of speeds. Each zombie has a slightly different randomization of speeds. Spawn them in with clothes based on their race/class. The clothes should be visible when you examine them. They also should mostly not be in pristine condition. The special zombies I feel should be moved to locations other than the city. They should be made nastier and made more of a big deal when you find them. I think special zombies should be moved to labs and other odd locations. I think the tanks and hulks I should be busting out a frig’n rpg or running like a lil girl. If there is a way to do it, how about a creature that is more then one square? Tanks being four squares and not easily fit through doors. But able to bash through walls (take damage from it!)!
Zombie Bites! Your character should generally be considered immune to the zombiefication virus. As you don’t react to any of the bites. So bites shouldn’t immediately zombiefy you. But still, being bit by a rotting corpse doesn’t sound like the most cleanly of thing. So having minor debuff of zombie virus after being bitten wouldn’t be a bad idea. If you let your immune system get low by being out in the cold and not eating, drinking, or sleeping enough… well… that might be another story.
If I have anymore thoughts I’ll post em up later.