1 - Server sizes: After playing on some private servers I really, really hope they bump the server sizes (maybe to 4-5k). I think true Vanilla numbers were around 2-2.5k
2 - Dynamic respawn: If a group of people are farming for a quest mob/item having a dynamic respawn would be pretty amazing!
3 - Allowing us to create Horde and Alliance characters on the same PVP server
Just my opinion and I think a lot of people will agree these changes would really make Vanilla better but also keeping the spirit of Vanilla alive.