From what I've seen recently most people seem to think that we are heading towards a legion expansion.
I've thought about it a lot and this may have been in the works for longer than you might expect. As far back as TBC.
The first raid instance in the burning crusade was Karazhan, the tower of the Guardian Medivh. The Kirin Tor sent a group of agents to investigate the tower due to a recent disturbance. This group is known as the violet eye. The violet eye send you on a quest chain to create the masters key to grant you access to the tower so that you can enter and put an end to the demonic presence there. What is interesting is that the final boss (Prince Malchezaar) can drop Grom Hellscream's axe, Gorehowl. It was never explained how he got the axe in the first place but at the time it didn't really matter.
Fast forward a few years and the character of Garrosh, Grom's son, has become an extremely large part of the story. In the book "The Shattering" Thrall gives Gorehowl to Garrosh slightly before he makes him Warchief of the Horde and as far as we know Thrall has had the axe since Warcraft III when Grom died.
So how can both Thrall and Malchezaar have the axe at the same time in the burning crusade? Well what if it's not the same time at all? What if Prince Malchezaar gets the axe AFTER Garrosh loses it at the end of MoP?
Notable quote; Prince Malchezaar - "All realities, all dimensions are open to me!"
So how is that possible. How could Prince steal the axe from Orgrimmar then travel back in time to burning crusade and hide out in Karazhan. Or rather why would he do that. I don't think that is what happens at all. It is not Malchezaar who travels through time; it is us, the players. I think that Medivh is still alive and is currently inside Karazhan working to unleash the burning legion upon the world once again as Sargeras made him do with the original dark portal. Karazhan is located upon a "nexus of ley lines" which is part of what gives it its magical properties. This would be the perfect location for a mega-portal between two worlds. Medivh, still under the influence of the demon lord Sargeras, may have put a magical veil over the tower of Karazhan such that when you enter it in TBC you are not entering the tower as it is then, but as it is years into the future (post Siege of Orgrimmar). One of the possible responses you can get when attempting to open the puzzle box of yogg-saron is: "The giant rook watches from the dead trees. Nothing breathes beneath his shadow." To me that sounds like a direct reference to Karazhan.
The burning legion have learned from their previous invasion attempts that in order to successfully conquer Azeroth they must first weaken the resistance of the mortal races. This was the original purpose of the Undead Scourge before Nerzhul's betrayal. So what better time to invade than immediately after the Warchief of the Horde has been killed? This may have been Sargeras' plan for a long time. In Wrath of the Lich King Varimathras betrays Sylvanas and the forsaken apothecary Putress unleashes the plague upon both the horde and alliance yelling "Death to the scourge and death to the living!".
What follows is the battle for the undercity quest where you work your way towards Varimathras and end up killing him for his betrayal. Varimathras makes reference to "years of planning" (attempting to undermine Sylvanas) and the voice of an unseen "Master" is heard berating him for his failure, "You have failed me Varimathras." This voice is marked as Sargeras in its sound file. So it is confirmed that Varimathras' rebellion was because of orders from the lord of the legion himself. His plan was to further break the trust between the horde and the alliance and start another war between them. The only reason why Archimonde failed in his attack upon the world tree at the end of Warcraft 3 was because Medivh, his good spirit, convinced Thrall, Jaina and Malfurion to rally their forces and stand united against the Legion to hold off the attack.
I believe that the good spirit of Medivh is at war with his still "living" phyiscal body and is trying to stop Sargeras' plans. If Medivh is still alive and is living in Karazhan then why can we not find him in the raid? This is because he is in the other version of Karazhan, also known as Lower Karazhan or "Upside-Down Karazhan". If this were to become a raid in the game we would not ascend to the top of the tower but rather descend through the lower parts of the tower into Medivh's secret chambers. "When Karazhan was first announced, there were rumors that an upside-down version of the tower existed, extending down from the bottom floor, deep underground." In game Archmage Alturus tells you that Archmage Alvareux wrote a treatise, proposing the existence of an exact copy of Karazhan in another world that somehow shares the same space. I think that to enter this version of Karazhan we must enter the tower from the top and not from the bottom. There exists currently in game a portal that is gated off at the top of the tower that has so far served no purpose. It makes sense to me that to enter an upside down version of a tower you should try entering it at the top.
If a legion expansion is indeed the next one I think that Karazhan 2.0 will be one of the raids with the final boss being Medivh / Sargeras himself. Perhaps in the room before the final boss room you come across Medivh and he asks you to wait because he is in the middle of a game of chess... A game of chess that he is playing against a group of heroes of from the horde or alliance that have travelled from the near past to his tower He will of course go on to lose this game of chess however Sargeras knows that in war losing one battle can sometimes place you in a better position for future battles.
Finally we look at how the dragon aspects play into this theory. In the End Time dungeon Murozond, a corrupted version of Nozdormu, says "I have witnessed the true End Time. This? This is a blessing you simply cannot comprehend." What could possibly be worse than the eradication of all life on the planet at the hands of the old gods and Deathwing? Maybe the entire planet's (and soon to be universe's) destruction at the hands of the legion. It is the duty of a dragon aspect to do whatever they can to protect the planet. In his own twisted way Murozond may be fulfilling his purpose given to him by the titans. Malygos may have been aware of what Medivh was up to in Karazhan and perhaps started the nexus war in an attempt to stop it. "I had hoped to end your lives quickly, but you have proven more... resilient then I anticipated. Nonetheless, your efforts are in vain. It is you reckless, careless mortals who are to blame for this war. I do what I must, and if it means your extinction.... then SO BE IT!!"
If you have any ideas that either support or contradict this theory I would like to hear them.