Going "What a twist!" is not exactly good story telling when the twist is out of left field and has nothing to do with anything we've been doing on Pandaria.
because right now, it'd be like this:
MoP:
-Wrathion eluding to the Burning Legion's invasion
-Wrathion flatly saying "the burning legion is coming
-Wrathion making a heartfelt appeal to the August Celstials for powers to help fight off the Legion
-The burdens of Shao-Hao detailing the emperor's lesson needed to protect Pandaria from the legion, and the lessons needed to be learned by the Pandaren people in order to defend themselves from the Burning Legion in the future.
-and then the big bad we have to contend with in the next expansion, of course, is...
The Burning Le- JUST KIDDING! It's N'zoth!
So it's not the most... sensible... story avenue.