Something that came to mind when i read The Dark Below was Karazhan - We know how Bliz love to use old locations as new content and this location could fit in with some people expectations of a legion involvement. We know that Kara had ties to the Legion in the past with Medivh and Sargeras, Blizz could also use it as a reason to finally explain why Malchezaar was there.
Quote from wowpedia
Archmage Alvareux wrote a treatise, proposing the existence of an exact copy of Karazhan in a world other than Azeroth and that both somehow share the same space, which might be a reference to lower Karazhan.
Obviously this is a stretch of the imagination but thought i would chuck it out there. If the above quote is correct then we could be looking at traveling to a paralell Azeroth - That obviously opens up a lot of content.
But what if we stretch The Dark Below a little further it could relate to the Great Dark Beyond which has ties to the twisting nether and again more demons ^.^
But as i said above im stretching things for the sake of fun. It is more likely to link in to Azshara due to Ozumat in-game tag.