Literally anything, but I think the main points are-
1. Fix bot crisis (or ya know, update the anti-cheat or something)
2. Add some new weapons or maps
3. I don't play that much MvM, but I think adding new maps to that would be fun
In my opinion, I think that valve should switch to releasing small monthly updates instead of major updates every 3 years. The small updates would be a lot more manageable, like adding in 2 weapons or a map, which is a lot more than 6 maps, 6 weapons, cosmetics, etc.