There are indeed high cliffs. If you “ghost” through them, you can see the volcano dungeon after a while. But there seems to be quite still a huge space between the cliffs the outer edges of the dungeon.
The cliffs:
From here, you can see the volcano dungeon:
This is how it looks:
The Warmaker’s sanctuary is here:
The Wine Cellar is here:
The Dregs:
The Sunken City:
So, as we can see, there’s plenty of room.
Also, somebody said in the forum that the verticality of the map isn’t used well. Apparently, everything was put on the same plane, while the map is actually a cube.
I don’t know if you can “drag and drop” dungeons in the UE4 engine, but if so, they could just be moved far beneath the map. Didn’t one of the experts say that one could put the whole Isle of Siptah in the Exiled Lands map by just putting it up high in the sky?
Additional dungeons can also be added, as Snowhunter, Darg and others have shown, by using this space. Desolate Lands adds, among others, the “Festering Crypt”, which is physically located here:
IIRC, it’s really just console limitations that stops Funcom to expand the map from a technical perspective, and that probably was never really tested, because they don’t have the time/money/motivation to do so anyhow.
I’m not a tech guy, so please excuse me if I say something stupid here, and take it with a grain of salt.
Btw.: If they’d crowdfund some official expansions… sigh Some of the great modders could probably be persuaded to offer Funcom something, just as Snowhunter was contracted to do Wine Cellar and Warmaker’s Sanctuary.