A black hole launcher would be cool, even if they kinda did it in Unreal 2.
Now as for absolutely zany, I would like a gun that puts a target beacon on your foe and then random lethal stuff happens. For example a safe falls on his head out of nowhere, a giant cartoony hand comes out of the sky and blasts lightning up his ass, a Killer Rabbit tears his throat out, grenade suddenly blows all his clothes off thus causing him to flee in shame, eaten by Godzilla you know, absoslutely crazy stuff.
Just think of every single Looney Toons cartoon you can remember, I'm not going to get a bunch of blank looks when I say Looney Toons, am I?