He's not bringing a Sha in a box to Orgrimmar. Shaohao never gave up his Pride and so the Seventh Sha remains within him.
I never really understood where the idea that Garrosh would be empowered by a singular Sha came from either. The whole theory of Sha-Garrosh was born out of how well he managed to fit with all of the Sha and through them he could become something more. More than any singular Sha would be. We've defeated so many of those already.
Anyhow, what's in the box isn't a Sha.
It's Y'Shaarj's heart. The core of his being entrapped within a Titan device. An organ if properly utilized could almost certainly provide one with mastery over the Sha and all of its power. Perhaps, if one was a suitable vessel, one could even take in its power and become as a God themselves.
One would have to be able to properly embody all of the negative emotions present in the Sha, Fear, Doubt, Violence, Hatred, Anger, Despair and Pride. To wield these as a weapon and make proper use of them.
One could even say Vol'jin may have foreshadowed this matter.
And lo and behold, our good Warchief Garrosh has claimed for himself a Black Heart. Also there's the point where chances are his name translates as "Heart of War".Vision of Vol'jin says: Ya will spend ya reign glancin' over ya shoulda and fearin' tha shadows, for when tha time comes and ya blood be slowly drainin' out, ya will know exactly who fired da arrow dat pierced ya black heart.
Who better to claim the power of a literal "Black Heart of War" than Garrosh? It almost seems like destiny...
Well Metzen did say the enemy was "war itself."
"I bring light where there is darkness. Hope where there is despair. Judgement where there is injustice. I AM PALADIN!"
And truly, can you think of any greater glory than to master a God's power, to become one with it and to provide it with life so as to join it in divinity?
On Pandaria Garrosh discovered the Mogu secret of shaping life. As a God he shall have those who will serve him and be able to shape them to his will and create those living things he desires. He will be able to mold life in his desired image and set it loose on the world as well as bless his followers with his might.
Under his direction the Horde could rise to new heights of power and strength to forge a new Azeroth. And perhaps it is his fault that the Horde now falls apart, but that is more due to the other leaders lacking his unique vision.
Needless to say, I believe Garrosh will leave his mark on Azeroth even after his defeat. A stain that will not so easily be wiped away.
And who knows? Maybe he'll even survive the siege, though banished from our realm, as an aspect of Y'shaarj or through the assimilation of his power.
And there are likely to be followers of his that would survive to bring his ways to those who would listen. Converts perhaps drawn from the Burning Blade, Shadow Council, the Blackrock and perhaps what little remains of the Twilight's Hammer. But of these elements I feel the Blackrock may be the most relevant.
Rend has been dead for some time. There's ample room for a new, more sinister, Dark Horde to arise.