A zombie MMO would not work, in that it would be a victim of its own popularity. The zombie is to be feared, not because zombies are frightening - one zombie is inferior to one human - but because of the sheer weight of numbers. That is where their strength lies. The problem is, if you have lots of human players, all of which capable of taking down hundreds of zombies, the zombies are no longer a threat.
Yes, alright, you can have people playing as zombies - except
most people will not want to. It's the ultimate nerd survival fantasy - the lone Tallahassee style badass, cracking one-liners as zombies get chainsawed. Hundreds, thousands of these characters, and what kind of a threat do zombies pose? They don't. In zombie movies, there's always a bunch of unlikable whiny characters who get eaten quickly. If there was a real zombie apocalypse, the vast majority of the people who indulge in the zombie fandom would fall into this category. Probably all of you reading this thread! I don't give a smurf about your special zombie plan, you'd be eaten. I'm a huge fan of the genre and I have no delusions - I'd be zombie chow. And for a real zombie apocalypse, that's what you need - hundreds of chumps being god damn massacred, otherwise it's not an apocalypse. Nobody wants to play as the chump, they want to play as Tallahasee.
Let me give you an example of what I mean. There's a browser-based zombie MMO called Urban Dead.
(Link) All the zombies and humans are player controlled. Humans go from building to building, scavenging supplies, building barricades and whatnot, and the zombies eat. There's also a mechanic where zombies can be revived into humans using a NecroTech (read: Umbrella) syringe, so death is never permanent. The simple fact is, the overwhelming majority of the player base did not want to play as zombies. This led to pretty much the entire city being a fortress for humans, completely powered and safe. Humans feared other humans more than they did the actual zombies. They sat around in their castles, roleplaying and acting like badasses. This carried on for years - the zombie apocalypse was a zombie mild inconvienience.
Only recently due to a flood of people from the usual sites (Something Awful, Shacknews, LUE etc) playing as zombies led to their nice suburban existence being destroyed. Of course they all got smurfing slaughtered like the chumps (see above!) that they are... and they are not happy. Do they play as zombies? Do they smurf. They sit around doing nothing for
weeks waiting for someone to revive them as a human. They whine and complain to the creator of the game that zombies are overpowered (they're not; quite the opposite due to previous complaints!) because they got killed by a zombie in a zombie apocalypse game. And it all goes back to what I said - Nobody wants to play as the chump, they want to play as Tallahasee. And thousands of Tallahasees vs zombies is not an apocalypse, it's not about survival, it's a turkey shoot.
So yeah, zombie MMO? Wouldn't work. Now, a zombie game in a GTA-style free roaming city, with up to 16 players? That would work a whole lot better, imho.