I see a lot of speculation threads where people declare that defeating Sargeras would be the end of WoW. But I think there's an even bigger menace out there that could come to end Azeroth, particularly if the mortals had banded together to wipe out the Burning Legion: the Titans.
Killing the fallen Titan would cause his brethren to take a closer look at Azeroth. They'd find that everything they had put into place has been shot to hell. Algalon covered for us once, but now that they've seen us they worry mortals will be an even more destructive force in the universe. Re-origination is the order of the day.
That doesn't sit well with anyone who calls Azeroth home. The Alliance, Horde, Cenarian Circle, Argent Crusade, Earthen Ring, Ebon Blade, even the Dragonflights band together to fight back. Initial instances and raids will visit familiar Titan related points on Azeroth. The final tier will be an assault on their citadel. Thrall kicks of the raid with an epic speech, and we go beat the Titans until they promise to leave us alone.
Their hearts weren't really into the whole “obliterate the bravest races in the universe” thing in the first place, of course. Now they draft us into facing the next expansion's big bad, because the epicness of the expansion proves WoW is still a cash cow and cannot end yet.