I 'd love them to add the nifty "See friendly ammo count" to the Medic's core mechanics that is present in MVM but no in normal play.
So it would become possible add a downside to the Amputator and Solemn Vow being "Cannot see friendly ammo count while healing".
And a new bow for Sniper.
And a fix to the Mini-Sentry not having a smaller range, respecting the 3D Sphere (detection area) being smaller then the stock. And a new Engie PDA!
And a new Demo set with explosives!
And... Let me think about it. I'll come back to you guys.