Now i know we are all convinced the next x-pac will be about the legion, even though personally i think Azshara should be dealt with first. But, why couldn't it be both. Azshara summoned the Legion before, she could easily do it again.
I base this all off one quote from the Zandalari lorewalker info. Zul speaks of his three visions, ones of Deathwings emergance, one of Zandalar sinking, the other,
'Months later Zul returned, bearing more grim news from his visions. He saw a legion of serpents pouring forth from a gaping fissure that tore open the floor of the ocean.'
Now that, that hasnt happened yet. Those serpents are clearly Naga, and that crack could infact be either the maelstrom or the Rift of Aln.
Combining this with the Pearl of Pandaria (a mysterious artifact the naga wanted), and the fact that Naga had been looking for Pandaria, maybe will be seeing Azshara this expansion or more likely, in the next one.