FWIW, my initial ideas on the possible 'fun' rooms:

Start
- replace the airlock style start of allinone with a small 'assault course'.
Nothing too big, maybe just a couple of climbs, and jumps, leading to a platform accessed by a ladder which houses the buttons for area selection.
- kinda like a small version of the assault course Lara Croft has in her garden, deisgned to narrow as you approach the final ladder; but kept as small as possible so as not to become annoying at round start.

Knife
- Futuristic arena which keeps getting smaller (i.e. walls move inwards). Chuck in a couple of fun bits and pieces (ladders, moving platforms, etc) and maybe 'spike' the walls - sort of Mad Max meets the Killing Floor (from Johnny Mnemonic - d'oh I've gone all 'Gibson' again)

Grens
- not sure really, but would like something a little more interesting than the usual "tennis" style map- maybe some platforms, or some moving things to bounce grens off.

Pistols
- gotta be a Wild-West style shootout, maybe restricted to duals

Love Sput's idea of a shootout on a moving train and a zero g area.
FWIW, I reckon each area should have a very distinct "theme".
Not sure whether this could be achieved using std textures (with a couple of custom ones for the labels, etc); or whether it would be feasible\worth it to design new ones.