Spoilers abound in here for both War Crimes and WoD quest content. You have been warned!
So I've had this theory for a little while now and I'd like to see what others thing about it. I'm sure everyone remembers Wrathion's prophecy way back in 5.0 when we started the Legendary Cloak quest chain. He explained that a great evil is coming and it will wipe out Azeroth. Judging by the image of Infernals raining down from the sky, it's probably safe to say he's talking about the Burning Legion, I think most people agree on that assessment.
Throughout the expansion, he allied with both sides in the Horde vs. Alliance war, though we learned in 5.4 that in secret he was backing the Horde to win as the stronger side, right up until Garrosh went psycho-bonkers-crazy and caused a civil war within his own faction. At this point, Wrathion switched to backing the Alliance until Varian let him down by not attempting to overthrow the Horde leadership after the defeat of Garrosh. This leads Wrathion (in War Crimes) to release Garrosh and kickstart the entire Warlords of Draenor expansion by using Kairoz to free Garrosh and send him back to Draenor to form the Iron Horde. Either Wrathion believes the Iron Horde would make better rulers (and thus protectors) of Azeroth for the coming troubles ahead than a divided Horde and Alliance, or he believe that a common enemy like that will bring Horde and Alliance together into one united force.
That's what we know so far.
I'm beginning to wonder though if Wrathion's prophecy of a Burning Legion invasion will turn out to be a self-fulfilling one. Obviously, the Iron Horde are going to be defeated in WoD (we already know from 6.0 alone that the vast majority of the Warlords will be eliminated, culminating in Blackhand being killed in Blackrock Foundry at the end of the raid tier). I think it's safe to say in future patches we're going to deal with Tanaan Jungle, the Black Citadel and Grommash. And we're still going to be fighting non-stop in Ashran. So whichever version you bet on, Wrathion's plan is going to fail. Some have assumed that this will simply mean he will get angry with us and end up going insane or whatever and somehow being linked to the end boss of the expansion.
What if it turns out that Gul'dan - who we know has already been in contact with the Burning Legion in AU Draenor - escapes back to our MU Azeroth and is the catalyst for summoning the Legion to OUR Azeroth. This has been something a lot of people have speculated, I know. Seems like a likely possibility, since Blizzard mentioned that WoD will lead directly into the next expansion.
But as a side effect of this, it turns out, that by sending Garrosh back to the AU so he could build the Iron Horde and defend Azeroth from the Burning Legion... Wrathion actually facilitates the return of the Legion by allowing Gul'dan to escape to our universe. The Legion would never have been able to get back to Azeroth in force (any time soon at least) if Wrathion hadn't inadvertently released one of the most powerful Warlocks we've ever known back into our world.
Can you imagine Wrathion's reaction upon realising this? Everything he's worked nearly his entire life, since Deathwing was killed at least, has only managed to cause the very tragedy he's worked to prevent? Perhaps this will finally be the kick up the arse he needs to realise that the end doesn't always justify the means, and he'll finally become a force for good? Or perhaps he will be driven insane, and become a raid boss after all?