Spy does not need a buff in the essence of giving him more strengths. He can already be quite impactful, just not in every situation. He is a support class that is supposed to turns things in your favour. But in 6v6, it can be pretty difficult to land a backstab on somebody because the enemy team is not always distracted when a Spy pick could be helpful.
You could give him ways to distract the enemy team without relying on his team; decoys could be cool. However, people may find that unfun to play against. Still would be cool to see if it works or not though.
I think the easiest buff spy could have is to decrease the time it takes to cloak in and out. So escaping is easier, and timing your decloaks is more effecient. Added with a 'Silent Killer' stats being put on all knives.
A more ambitious idea I have is being able to actually 'mimic' other classes, instead of only looking like them. For example, you could disguise as a Soldier and actually do everything a Soldier can, without actually doing anything substantial like damage and frags.
To what point you can 'mimiç' is up for debate, would you be able to rocket jump too? Or only shoot out projectiles?
Anyway, I think Spy is in an okay spot right now. But the Spy could definitely have a couple of decent buffs that also do not make him into a very oppressive class.