I would say if I could mandate one nerf it would be the vaccinator. It isn't as overpowered against competent players as some items (actually it's quite bad in organized competitive), but in casual you can completely shut down a single player on the other team with it. Nothing feels worse than being the only sentient player on your team only to have a vaccinator medic render you essentially useless. Sadly the best way to counter it is to get your own vaccinator pocket, and the game just devolves.

I honestly do NOT want a TF3 based on what happend with CS2, but if it were happening I would want to see all of the classes to be given items that allow them to be more fast paced without breaking them. For example, give engineer an alternative to his default teleporter, which is very hard to get value from on 5 control point maps because of how quick and dynamic the mode sometimes is. Give heavy something different that isnt broken too. I'm not sure how to fix these classes to make them less repetitive, but that would be nice to see in a theoretical tf3. They would need to keep a LOT to make it worth playing. I would say mimicing the movement is the most important, even bugs like speedshots and trimping. I am by no means a programming expert but I am assuming that this would be near impossible to get right which is another reason I don't really want at tf3.

Lastly, I think there are already spin off games that I already want the most, such as Open Fortress and Team Fortress Classic that will be able to be put onto steam soon because of the SDK being released. In a perfect world i would like to see a competitive mod released as well that fixed some issues with existing gamemodes like MGE (tf2's community made mod). The in game "competitive" is terrible.