Sorry if someone posted my idea, but I didn't bother reading the posts here. Sorry.
Basically I based this idea off the new knife in TF2, because it's exactly like it. Funny enough I didn't base it off of it, but when I learned about the new knife it was exactly like it. So I was lke :/
The stab sound? Remove it, but only when a successful stab has been done. If you fail a stab, the sound will go off to alert a spy is around, but if the stab was successful there would be no sound.
Not to mention there would be no death noise or animation. (Sorry backstab animation people) Upon death the death-cry would be silent, the death of the character wouldn't exist, meaning he'd just blip from existance, and the stab would be silent. Now you'll become the person you just stabbed, and because of the passive invisibility no one would be the wiser.
I'm unsure how you could stab in this state, because if you make right-click stab then you lose the ability to cloak which is stupid. Perhaps it could be middle-mouse button.
Anyway if you stab while diguised you'd simply stand still for the alloted time it takes to stab, so there'd be no animation, and then when you'd stab to become the spy it would check.
If failed: you would un-disguise and become a spy again. The stab sound would activate, and you would go into cloak.
If successful: You would become the person you just stabbed.
This way we're implamenting "disguises" which everyone wants, and the spy gets a buff.
there we go, that's what to do. DO IT
So many people have said this it's funny, but that doesn't mean it's a bad Idea. I've said it myself on multiple occasions. I'd fund this idea.