If you've never played Super Crate Box, read
http://www.supercratebox.com/I was thinking we could use a small courtyard-type map where there are different places for the crates to appear, with random events/scripting. You have to kill the zombies that are coming out of a tube from the ceiling with your randomly acquired weapons.
Floating skulls are optional.
I think this would be a super fun multiplayer/singleplayer gamemode.