As I've heared at Blizzcon, Emerald Dream won't be an expansion, nor a single zone or dundeon - it'll be much like Elemental Plane in Cata - since it is a plane of existence, as well. So, perhaps, a zone per continent or so. That perfectly would come along with South Seas expansion. If seas, then Azshara, if Azshara, then Maelstorm and deep ocean. Who's deep in the ocean? N'Zoth and Ny'alotha. And if N'Zoth, then Nightmare, then Dream. See, Dream alone is not enough to make an expansion. Seas alone is not enough to make an expansion. But together - they are allies!
Just imagine, a world event - sun is permanently closed by clouds, rain streams endlessly, Ny'alotha rises from the seafloor, tsunami assault the bays, ships are wrecked by stroms, krakens and nagas, continents isolated, sea is dark and raging. From the dephts, to the grim, dark shores, endless army of nagas, faceless and unspeakaple underwater monsters stream to conquer the surface.
They brought new allies - the surviving Zandalari, followers of mad prophet Zul assist them with their dark arts, and Kvaldir, cursed pirates of the mists.
But, at the same time - weaks before the coming of the rains, lots of people are losing peacefulnes of their dreams, seeing nightmares instead. They grow depressed, zombified, and some begin to attack others while sleeping. Remnants of the Twilight's Hammer now form a new sect - Druids of the Nightmare. They are allied with the satyrs, who help the nighmare to drive wild forest beasts insane and agressive. Dragons of Nightmare betray their Dragonflight for promises of power and glory in the new dark world.
See how dangerous it is this time? We are all isolated at our continents, waters grows hostile. Not only we are besieged by armies of nagas, faceless and Zandalari, we are also weakened from within - the defenders cannot sleep safely, for Nightmare awaits to make its strike upon those who are foolish enough to fall asleep, to torture, bind them, and make attack their own allies. While the sea armies assault us from the shores, the armies of Nightmare, that include Druids, dragons of Nightmare, satyrs, maddened beasts, lunatics and yet unseen creatures spawned within nightmare itself, attack us from inside at the same time.
So, if the next expansion is all about this, I'd say it would be the most epic of WoW, because this would be a threat never seen before, more dangerous than anything we've yet encountered. What would mke it atmospheric -
1) All Azeroth covered it thick clouds, periodically raining.
2) Sometimes Azshara casts some meteospell at random zone, making hard rains, cyclones, and lightning storms, killing people (similar to Deathwing's surprise attacks).
3) Periodic alarms - ingame event, meaning an attack of nagas\faceless\zandalari at random city, zone, where we are to help npc guards.
4) Nightmare events - from the closest Nightmare zones come out strange nightmarish beasts, friendly npc's become mind controlled and attack players. Mobs also periodically drive lunatic, coming in masses, attacking settlements.
Two final raids of the xpac are Ny'alotha and Nazjatar. Nazjatar might be last and located underwater, while Ny'alotha - penultimate, risen above the seafloor, in the Dream's high level zone - Black Forest.
How about this?