Well I'm sure WoW will have more expansions, so you have to take that into consideration.
So what i suggest is the world of warcraft 150+ years after the current age of Warcraft, so none of the races(elves and undead excluded) would be alive in both games.
Like to see alot of the same races, but with minor changes, as with items, looks. You know, because over a period of like 10 years in america we went from hippies to nerds, than 150 years in warcraft could make a big difference.
Magic could basicly be reborn into the world. Because among the common races, human and orcs, magic is not extremly common ecspecially ever since dalaran got its ass kicked. It's just in the games where mages can be common lol.
New discovered landmasses? Newly formed towns and ruined towns of old(E.X. purely example. Ogrimmar got whiped by an Icecrown counter attack. So the Horde has a new capital, maybe the starting town for orcs, and trolls is prospering now, forget its name.)