We're really just picturing this as an offshoot of Left 4 Dead's versus mode. All players, except one zombie player, start as humans. As they die off, they add to the number of undead and take control of the attacking super zombies. You win if you are the last player standing.
Make it so zombie players have projectile attacks and buffed stats and it will be a good time. We've already seen this type of switching sides, infection gameplay from the other shooters, so it isn't too hard to imagine it with some zombies thrown in the mix.
Co-op can get frustrating with player controlled allies, but this would be your chance to get back at your incompetent comrades. Well, other than shooting them, that is.