As the post above states what does happen to Undermine?
Well Blizz has confirmed that it will be the starting zone for Goblins (at least for the first 5 levels) also it's setting will be a Grand Theft Auto type city.
We also know that when Mt. Kajaro erupts it would destroy Undermine (which is built right inside of the mountain...
Another important part of Azeroth that's missing is Kul'Tiras. It's quite interesting to notice that it has the largest navy in the world and is one of the last human nations on the Lordearon subcontinent but Blizz has not announced this to be a new zone...
Also Azshara won't make an appearance (as a raid boss) in this expansion which means she probably show up in a future expansion.
My Guess is if Azshara will appear in a future expansion with the focus on the Great Sea, maybe Kul'Tiras will become an Alliance base and Kezan and Undermine will become a Horde base.
If I am missing a piece of information please inform me.
I'd like to hear some other ideas
/discuss