The aetherial flows and the zonelines in CT/PS allow them to expand on the dungeons, as we can't tell exactly how large/small it is. You can see paths in Halatali and Darkhold too. AK has plenty of a areas that are just BEGGING to be opened up.

Who knows, maybe after SE feels a dungeon is as complete as its going to get through various difficulty modes, we might see that completed version in the open world as explorable zone.
PS:I wanna know whats behind that slightly open door in the wall near the Ixali Logging Grounds north of Fallgourd >_>