It's been said that Emperor Shaohao got rid of Doubt, Despair, Anger, Fear, Hatred, and Violence. And after that, became the mist that surrounded the continent for thousands of years.There is no mention anywhere of him expelling a seventh Sha. Maybe he brought it with him.
All right, hear me out on this, it's been said that the Mist vanished after The Cataclysm. Where has the Emperor gone to? Is it possible that, with the mists gone, the Seventh might finally manifest somewhere on Pandaria? Maybe this Sha is War itself? Something so powerful that not even the Emperor could get rid of it, so powerful that it requires all the other Sha to be at their highest point of influence/power in order to manifest, seeing as how the Alliance/Horde war does a great job at making them grow.
Just another crazy theory, would love to hear thoughts/ideas. Sorry if the post is a little crazy, I'm on my third cup of tea in under a hour.