Well it's probably with good reason that the bottom half of Mor Dhona is inaccessible. It was so close to the front lines of the battle at Carteneau that it probably got trashed pretty bad between the clashing of forces and Bahamut's onslaught. Anyone there on the last day before the servers went down has an idea of how it was with all the Garlean forces there.

Anyway, Yoshi-P said sometime during beta that ARR was going to be 40ish areas (including instanced areas) with the capability of adding 40 more through patches. So I honestly wouldn't be surprised if they expanded upon Coerthas or Mor Dhona. I'm kinda curious where that new Pharos area is going to be. Mor Dhona seems like a suitable candidate.