So at the end of MoP something has to happen. I don't feel the Alliance can just walk away and say, "Hey, Vol'jin, that was fun! But, you know, we're kinda in war and stuff. So we're gonna head out now." "Sounds good mon. See ya on the battlefield!"
That would just be bad story telling and not really make sense. If the Legion is on the way and we need to work together, wouldn't it make sense that an actual peace (not the fake peace that we started WoW with) but a real cease stabbing peace. This brings up questions like, What about PvP servers? What about BGs? Would Blizz redesign zones where Horde and Alliance are in combat? Would the Horde be welcome in Alliance cities, villages, quest-hubs, etc and vice versa? Would we merge the AH, too? Would this drive players away and lower subs even farther? Or would this be actual world change that would mean something and reinvigorate game play?