The Alliance could have let the Horde fight it's civil war and take advantage of the distraction it would have brought by strengthening contested territories. A majority of Kalimdor and anything aside from Lordaeron, Silvermoon, and possibly Mulgore, since we weren't 100% sure Baine would go all in with Vol'jin until he saw that the Alliance were backing Vol'jin, could have been the Alliances (I'm sure Sylvanas and Lor'themar would have put the defense of their own lands as priority). The Horde would have been in shambles after the Darkspear vs Orgimmar conflict, all the Alliance would have to do is clean up the pieces and slowly eliminate the now broken Horde race by race.
Instead the Alliance played the good guys this expansion, well, Varian did anyways. They not only dethroned the tyrant who was mad with power, but he put a new leader in his place so they could rebuild. This is a win for the Horde, and we're back to where we started before Cataclysm with both no side being good or evil (they'd have to try pretty hard to find someone as cruel as Garrosh).
After years of Varian despising the Orcs as well as the Horde, this isn't just surprising, it's outright out of character. We are either missing a key piece of dialogue or Varian knows something we don't. Varian was preaching patience earlier this expansion with the scenario in Krasarang, so maybe this was what he was waiting for, the Horde to destroy itself to avoid mass loss of life in his Alliance. He's also been much more diplomatic as seen in the scenario in Dun Morogh where he gives Moria a chance to display her loyalty instead of judging her based on her late captors actions, as well as the race she leads.
This doesn't make much sense seeing as Mists of Pandaria was supposed to be the catalyst to a whole new Horde vs Alliance campaign. Sure we've had much more conflict than usual this expansion between the two factions, but the entirety of the expansion we've had a common goal with no "war" happening. The only war we've fought was in 5.1 on the beaches of Krasarang. 5.2 was a race to who will unlock the secrets of the Thunder King first between Lor'themar and Jaina, the only battles we fought were because of how densely populated the island was, it was unavoidable. 5.3 had the Alliance actually helping the Horde rebellion, I still don't understand why Northern Barrens is a WPVP zone seeing as the Alliance are actually helping the Horde rebels. And with 5.4 the Alliance goes all in and sieges Orgrimmar, but they're not fighting the Horde, they're fighting whoever stands with Garrosh. This isn't a faction vs faction conflict, an extreme majority of the Horde separate from Garrosh.
And with the epilogue dialogue from Wrathion, he tells us of a coming threat. Now if this coming threat is next expansion that will just add to the pot of reasons why going back to the war between the Alliance and the Horde will make no sense. New leadership, a cause they literally worked together for, Varian having a change of character from his warmongering ways, the gratitude towards the Alliance for helping the Horde, and an invasion from a 3rd party. Unless the new leader straps Malfurion, Anduin, and Moria's child to a bomb, having the conflict resume will just be awkward.
tl;dr Alliance helped the Horde instead of ending it when they had the chance, causing everything to go back to normal except now there's an awkward one night stand phase we'll have to get over.
So what do you think will happen to cause the war to resume? If I'm missing anything or I'm just flat out wrong please let me know, because I'm stumped.