Icewall in western most Coerthas - I know it's currently impossible for anyone to breach it, but maybe someday.

The mine the guys are working at west of Fallgourd - This is likely just a place of interest but I wouldn't mind seeing it open up one day.

The invisible city in eastern Than - Would love the chance to dive into this area.

The floating city in outer La Nosc - Again, would love to get amongst that.

The list is vast and you can point out probably hundreds of places that could be opened up, but that's sorta the point for an MMO really... Never run out of content =P