A tower defence gamemode could be quite cool. It could work the current way Zombie Massacre does, where each new game you start off with 0 points and you need to learn positioning with your turrets and you upgrade your turrets or buy new ones at the end of the round (freeze/slow/fire/blast turrets etc).
There could be a few maps that decide the difficulty (less area for your to place turrets for harder or looping the ‘targets’ on an easier map.