I think it could be really great to do something around Thundar's and the High Moors, it doesnt necesarilly have to be epic levels, so long as its well done. If you look at the gnoll caves, thats a couple of zones that has a lot of atmosphere and for a quite good range of levels it has a high "replay" value. To have more zones such as this in the mid to high range of levels would be nice.
Theres a big difference to entering a cave/dungeon system and progressing through levels and layers, being pushed to the limits of your abbilities between rests (and possibly deaths). And circling an ogre/troll cave once or twice for 4 chests and leaving.