A grenade item that will temporarily fill the room with a heavy smoke that makes the difficult to see (unless you're rocking thermos/high perc/etc) people and objects in the room.
It would have a timer on it, so right after explosion the difficulty to see would be extremely high, then maybe on a 1 minute ticker it would slowly get easier to see maybe every ten seconds?
This would have a ton of fun uses from simple pranks to high stakes smash and grabs to sneaky PvP action.