How the fuck would they make WOW 2? Kill everyone off or something? Bring out a new foe? All that seems expansionlike =x.
WoW won't end the Warcraft universe, and I believe it's been stated that most events in WoW aren't considered Canon changes to the plotline.
There are still several commanders in charge of the Burning Legion, Arthas is (for now) alive, and Deathwing is still unaccounted for. Those are all old villains that don't really even need a cheesy "return" to be a threat (like having a cult resurrect Sargeras or something stupid).