'The Dark Below' makes sense to me..
Keywords: N'Zoth, Puzzle Boxes, "The fish know all the secrets. They know the cold. They know the dark." The dark below? Emerald Dream, Rift of Aln, Malfurion, Queen Azhsara, Siege of Orgrimmar where the Black Forest is mentioned, Ny'alotha, The Nightmare currently contained within the Rift of Aln.
"So he decided to collect energy from both Azeroth and the Emerald Dream to cleanse Azeroth from the Nightmare. Then he attacked the tree of Xavius in Azeroth while Thura struck down Xavius' shadow tree in the Emerald Dream. With his power source removed, the Nightmare Lord was defeated and imprisoned in the Rift of Aln.
The Battle against the Nightmare was over and mortals battling in the Dream returned to Azeroth. Exhausted Malfurion then awoke in Darnassus. Without wasting any time and with the blessings of Ysera and Alexstrasza, well wishes from the leaders of the Alliance and their friend and old compatriot Thrall of the Horde, the two were finally married in the capital city of the Night Elves, Darnassus."
- That to me, did sound a bit too easy. And Xavius was never the master behind it all.
So yeah, The Dark Below really comes at a good time, before any Burning Legion or anything of that matter.
Queen Azshara, N'Zoth, Black Forest, The Nightmare, The Emerald Dream, The Dark Below, down there in the Rift of Aln.
And if you add to that the last heirloom in Siege of Orgrimmar is 90 to 100 - I can imagine this be a large expansion with all of it.
Also, I don't imagine that dark below, down in somewhere dark, there is just water zones, no-no. Use more imagination than that.
Yeah, The Rift of Aln (Nightmare) is also believed by Druids to be middlin with the Twisting Nether, so it would be nice to actually take on Queen Azhsara, N'Zoth, Emerald Dream, Nightmare, Dark Below, Rift, all of that before middlin with the Twisting Nether and beyond (near) the Burning Legion, expansion.