It's been speculated for months now. We've been told in a vision that the Burning Legion is preparing itself for an invasion if Azeroth, and Alliance & Horde must unite to have any hope of defeating them. We've seen both factions doing more harm than good all across Pandaria as they continue to let their passions, wrath, and pride forego rationality...until they finally decide Garrosh has gone too far, and have teamed up to overthrow him.
So in 6.0, the war between the two sides will finally end, and they will be working together to defeat the next main villain, right?
I think not. Let's look at some key facts:
1. David Kosak has said several times before that PvP and the faction split in general is one of his favorite parts of the game, because once some competitive spirit is there it stays and makes things fun. Simply put, the war has become such a symbolic and integral part of WoW that getting rid of it entirely would require Blizzard as a whole to do a complete 180 in terms of design philosophy.
2. There was a blue post a little while back answering a question about whether or not we would see the Pandaren again in the next expansion, at least in an important role. His response strongly implied no, and that the main focus of the game will continue to be the Alliance and Horde.
3. All you have to do is look at Tides of War to see where Blizzard (and much of the community in general) sees "peacemakers". Just like with Jaina & Theramore, the whole story of Taran Zhu & the Shado-Pan is an exercise in frustration and hence with the destruction of the Vale, Garrosh has the last laugh.
4. You have to remember also, that most of the major lore figures did not even set foot on Pandaria, at least not beyond going there to partake in the war, so 90% of your factions' people didn't even learn about Pandaren culture and their concept of balance the way your character did.
5. And last but not least, although Garrosh may be at the heart of the strife right now, as well as one of the most evil villains in gaming history, he is NOT the only Spartan-type in the game by any means. I would say there are definitely plenty of other fanatical warmongers in both factions who are in control of enough of their faction to force skirmishes no matter what. For instance, Garrosh didn't truly become evil until he was made Warchief. Who knows what, say, Sky Admiral Rogers might have been like if she had been in control of the entire Alliance as opposed to just one platoon of it.
It is for these reasons that I am 99% confident that the Alliance and Horde will not learn anything from the experience, and that Mists was mainly intended to be a device to show players that a strictly peaceful race such as the Pandaren couldn't function in the Warcraft universe.
With Garrosh out of the picture, I COULD see the intensity of the war scaling back to BC/Wrath levels, but I predict that lore-wise, the Alliance & Horde still will not co-exist or tolerate each other's presence for the most part. At best, if the faction leaders do declare a truce at the end of 5.4, I think that Expansion 5 will actively depict how big of a struggle it would be to try and have peace between the two, and more likely than not, they will ultimately fail to keep the peace and it will end up falling to a couple of splinter groups to defeat the Legion, or Queen Azshara, or N'Zoth, or whoever the villain of the next expansion is.
I mean really, if the Darkspears' attitudes towards the Alliance PC in 5.3 are any indication...