Actually, I think you should choose from survivor or marauder when choosing a character. Remember in Dawn of the Dead? The looters on bikes? The survivors are more vulnerable to the undead, while marauders are more vulnerable to survivors.
Another aspect should be the bite meter. The health meter should be reserved for marauder damage/physical damage. But your character can only take one bite. After one bite, they've become infected. So the character only has one hour of humanity left. Sure, the bites to physical damage, and if the bite causes the physical damage to go to 0 then of course the character dies. The infected status is something that your fellow members know, and you'll know it too. Therefore, since you are BOUND to die, you get in the mindset of doing crazy stuff, like running out into the fields and taking out as many as you can. Therefore, your character gets mad stat buffs in the hour period.
Also, if a zombie should critical bite, it should go to a cutscene of zombies ganging up on your character and ripping his innards out, or crushing the eyes, or tearing the character in half (all ala Day of the Dead). Anyone nearby gets some of their stats lowered.
I also think that it's necessary to have a massive create-a-character system. This way, you get more than a standard 20 zombies. Think about it... with a massive creation engine, you could literally have hundreds of zombies that all look different, since all the PCs become zombies. That would add to the game tremendously.
Too bad this is all just brainstorming. Anybody get any connections with game designers?