Why would Ysera let us go there?
Good question, well let's see. The Nightmare was stopped last time because all races worked together in a massive Alliance and pushed it back, the planet itself worked with their essence and helped.
Now the planet is cracked. The races are in such a war that not even a common enemy can bring them together again, for example Deathwing, the Horde and Alliance still fought eachother even though both were in danger of being wiped out, same in MoP now with the Sha.
So, an etheral offensive against the Nightmare would be... too weak. So then, to save Paradise, you'd need to destroy it.
Besides, all evidence in pandaland so far points towards a shift of focus back to the Legion, so that's prolly where the next few expansions are gonna point.
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