Sounds like some good fun I'd play that, I remember the only thing that held back CoD zombies was the repetitiveness of the maps. If I were you i'd start off on a g-mod or minecraft build of it (both have zombies, map building and mods for the rest) just to see the core game-play in action before you start from the engine up. I'd love to see this happen, most fast to the action games lack a sense of creativity, so there's defiantly a place for this game.