the game programing doesnt allow for cool things like screwy physics, otherwise i would say an outer church level (full of Esherlike stairs) would be cool.
Counter-strike? Bah. You may as well make a Doom or Quake map.
I think Deus Ex and Max Payne would lend themselves pretty well to an invisibles fps, but I think Neverwinter Nights would allow for more creativity; it could actually have a plot other than "kill all the bad guys". The difficult part would be the skins.
if you want a game to mod to Invisibles-land... try Max Payne. The flow-mo time variables work, the engine's detailed enough, and I'm sure you could put in even more kooky physics.
