I think it would be pretty great if M1, did some sort of taunt kill. Not an instant kill or anything. Lets say a 2-3 second delay(like most taunt kils) where he pulls out his knife and stabs infront of him.

If it was possible to actually go into stealth with M2 would be hilarious. So you can go in and out of stealth in the taunt, while the movement is limited to the taunt-like tank controls.

Over-all none of those two things would be unfair i think since they are worst version of something the spy already does.