Next expansion theory!
In the Theramore scenario, Garrosh had made an arcane bomb, powerfull enough to destroy a hole city. My theory is about, that Garrosh is making another, just much larger... Much larger! Like the first 2 nuclear bombs. Then he would use the bomb in a final attempt maybe or in the last raid, where you get to fight, maybe kill him. That the bomb simply just made a rift/gateway to the Twisting Nether, out of the Energy burst. The would cause the "return" of the Burning Legion! Which would make the endboss, Sargeras! But he will not come through the rift/gateway, only his army and some of his officers.
Or he finds a sarced relic, maybe the package he was expecting as you see the start cinematic of MoP. (Hasen't much info about horde questline, as i haven't played it) That he will use it to do same an summon a rift/gateway to the Twisting Nether.
The reason that i think is going to happen, is that Wrathion says something is coming. And if aren't united, we'll die. Plus that the meteors you see in the first legendary cutscene, may be infenals falling down from the sky.
If this should happen, that would maybe conclude WoW, as Sargeras is quite a big lore character. As he's a titan, the Lord of demons and, makes all the other endbosses in wow, look like small beings.
Hope you enjoyed it, and write what you think about it