Apparently Blizzard has been modifying
the MPQ file that contains the Emerald Dream, which is leading folks to
believe it's probably the next planned expansion - it's a bit too big to be
considered patch content like Ahn'Qiraj, or (likely) Grim Batol and Uldum.
Head over to his site to get a sneak peak at more cool screens of these unfinsihed zones.