A zombie survival MMO wouldn't work that well, in my opinion. I mean, the general idea for survival in an actual zombie outbreak would basically involve long periods of staying held up in some fortified position while rationing supply's; not too much fun. The only way I could see it working would be if the world was set up with a virus that made zombie-like creatures, but created a much more varied set of infected as opposed to just normal zombies.
What if the games background involved a plague released from hell that turned people into demonic entities? Then the game revolved around angels from heaven coming to Earth to fight the evil. In this situation, you would take control of a certain class of angel, using their unique abilities to vanquish the demons. Or, of course, the game could allow you to control either holy or satanic characters, Earth being the middle ground on which they fight. In either case, I'm not sure how gear would work out.
What about a Fallout MMO?