If Blizz were to add the Emerald Dream, you'd be fighting The Nightmare (whatever it is). It's the Nightmare that trapped Malfurion in the Emerald Dream, and it's the Nightmare that has been driving the green dragonflight insane and corrupting the Emerald Dream. Malfurion and Ysera (the leader of the green dragonflight) have been working together in the Emerald Dream to hold off the Nightmare. There are numerous mentions to it in the game if you take the time to actually read what the NPCs are saying.