A problem with mirrored zones is that we have a finite number of zones. Not counting the cities, both HW and SB added 6 zones. We have already seen 3 zones for Shadowbringers. And these 3 really don't look like zones that are connected. They are just all over the place. Filling the other 3 slots with dark versions of these places, would just make the new zones less connected. Even though dark pixies and dark viera would be fun.