This is Pure imagination. Not Speculation or anything like that. But there's gonna be a lot of writing that has gone into this. Please, hold your applause until the end.
The battle for Orgrimmar is the setting.
Garrosh has gone crazy, absorbed the power of the Sha and has become nigh invincible, rampaging and slaughtering people in Orgrimmar, the Kor'kron also Sha infested are helping him, players fight the Kor'kron up to the last boss, being Garrosh, who is locked in combat with Varian Wrynn, who gets cut down as players join. Players would fight him with the help of the other leaders of their respective faction. As You whittle Garrosh's health down, he becomes more and more powerful, and more of the leaders join in. At around 50% the Dalaran faction teleport, once again, for their respective factions. Silver Covenent, Vereesa Windrunner and her troops, or Archmage Aethas Sunreaver, and the Sunreaver troops, and tell you they've come to help and they have to hold off Garrosh just a little while longer because Jaina and Rhonin and the Kirin Tor are on their way.
Once Garrosh's health gets to, say, 10% the real shit happens.
*Queue Cinematic cutscene*
Garrosh roars with rage, bleeding Sha essence, his eyes smoking and black smoke-like flames leaping from his shoulders. He looks up and sees Dalaran floating above Orgrimmar, and the Kirin Tor are preparing a spell to destroy him and the Sha completely.
*queue giant Laz0r Beam of uberl33t magics*
The Immense powerful from such a magic event draws the attention of the Burning Legion.
And The sky turns black as a demon 'Harbinger' tears through from the twisting nether and tells the leaders that the Burning Legion is on their way, and there is nothing they can do to stop it. And then teleports away.
And then Blizzard fills one more patch of people preparing for the invasion. hereby dubbed as the "Abyssal Legion".
You do some quests and stuff and find out that, the "Abyssal Vanguard" us here and they are wreaking havoc, and preparing the staging area. And preparing to Summon Sargeras.
At this point Tirion Fordring, meets with the Kirin Tor and they decide the best course of action.
The Leaders of both the Horde, (now once again led by Thrall), and the Alliance, (Varian Wrynn was killed fighting Garrosh and was replaced by Anduin), The Argent Crusade, The Sunreavers, The Silver Covenant and The Kirin Tor agree that they need to contact the Guardian of Tirisfal and the New Council.
So Med'an shows up and tells them that only a limitless army can beat a limitless army. Thus leading players back into ICC to free Highlord Bolvar Fordragon, the New Lich King, to ask him for help.
The last expansion gives 10 levels (bringing the level cap to 100), gives players the choice to choose Horde or Alliance regardless of race, and features both the Scourge (Friendly) and the Abyssal Legion (Hostile). And plenty of other things. >.>
I'm sure there's plenty more that can be built on here. (Sylvannas and the Scourge, Med'an and where he's been, how Pandaria is different now because there is no more Sha, etc.)