Hi
I think a virus type sapper would be interesting. It could do a couple things..
When attached to any object, other sentries see the device as an enemy object and shoot at it. This would be great when having to deal with a team with multiple sentries. Attach to one and watch the others attack it. Being able to attach it to players with the same effect would be cool by me too.
When attached to a dispenser, it makes it do the opposite and start taking ammo/health away from the players. This seems a little meh, sure someone can think of something better.
When attached to a teleporter exit, the entrance still works but results in the player being killed when they take it.
I kinda think it would also be nice if you could disguise as the last player you back stabbed. This would give you 10-20 seconds to run around as the player while they are waiting for respawn.
better taunts would be awesome too, the existing two are pretty lame. I dunno, maybe take a hit from the cigarette, blow a buff of smoke and say 'was it good for you? cause it was good for me.'
-ack