The wunderwaffe may be a pain to script and to model but the winters howl/Fury would be interesting... the elemental spells pack already has a freeze script...mabye its possible to lengthen the freeze time and make it insta-kill when the ice is touched by a player or shot...(would not freeze a player...just slow them down a bit)
This and that gun that makes the zombies into a random civilian that runs screaming would be interesting to see...
Don't know what im talking about? look up call of the dead, then wonder weapons.