It's not going to happen, the development team made that clear back during the Beta - on the long defunct Beta forum they were asked if there was going to be open world dungeons added and they categorically said "no". If it was on their agenda, I think they would have said something by now. It's time to face facts, instanced dungeons are here to stay and you are not going to see open-world dungeons again in this game. That's just being realistic.

Besides, if you want to explore a dungeon, just under-man it at level 60 and run around the place to your heart's content (or at least til the timer expires ).