There's a lot of retcons I'd like to see, but if there's one that I really want to see badly its probably that the forsaken and night elves were their own faction. WoW could have started like this:
Alliance:
- Human
- Dwarf
- Gnome
Horde:
- Orc
- Troll
- Tauren
Forsaken:
- Undead
- Blood elves
- Demons (lead by Varimathras)
Night elves:
- Night elves
- Worgen
- Naga