Completely random idea. Let a player be the director. he/she gets a amount of resources like. 1 tank, 1 witch, 3 of each special and maybe like 45 zombies. While the game is going they can drop (from like a either a player perspective or F8 perspective) these types using commands or key binds or something and then that AI or player would spawn. Once the resource of zombies run out the survivors ,are home free and stuff. Kind provide that RTS feel and completely something new.
I realize that this probably isn't going to happen and I don't even actually know if this is possible but, I'm just chuckin Ideas around.
EDIt: Use the number keys to spawn the specials. Also, still have Ai put items and stuff if it does that I cna't remember it's been awhile since I've used it.