How bout this. Zombies cant fly.
Then you're limited to a flat plain map or no interaction whatsoever. I like the idea, but good luck getting people to distribute teams correctly. It's bad enough just trying to get an even split across two teams at times. Plus, new people joining or just-turned-zombies quitting will offset the games often I'd think, as most people won't be willing to tolerate having to play with just melee attacks.
