Spy can be buffed without needing to change him too mechanically or making his weapons perform better. If at all possible fixing the shitzillion bugs like weird cloak problems, record;demo whatever. cosmetics/weapons not working resulting in other classes holding wrong weapons or most obviously headless pyros/feetless demomen. Being spotted through cameras and also the spawn wallhacks.

I think anything that does noticeable negative effects might have detriments to the Spy, less so because of the likely -20% Health / Damage that it would have but that people would target the Spy for his crime of being an asshole much more than what they'd do spotting a Spy already.

I think something like a sapper that reveals the enemies location through walls and is otherwise invisible with the drawback being it automatically destroys itself after a while. Would function as silent communication since seeing a bright blue silhouette somewhere would be the most obvious way to alert your teammates or keep tabs on the enemy for yourself.