Well, Oblivion gates were pretty much exterior dungeons for all intents and purposes, and the towers in them were basically the inverse of what most dungeons do. Whereas typically with dungeons you enter at the ground floor and if the dungeon has mutiple levels, the direction you want to go is usually down, with Oblivion towers, you're climbing up until you reach the Sigil Stone. In Morrowind, the Dunmer strongholds were typically above ground for the most part, they might have tunnels under them like sewers and such, but a lot of the interior area of such places was located in the above ground structures. So certainly, in past games, dungeon exploration has generally been underground, though there were some exceptions, I wouldn't mind a few more above ground dungeons in Skyrim. I suppose tradition aside, the main reason why they're usually underground is because it's just easier to that way, as you can make an extensive underground complex without taking up much space above ground, but I'm sure there's somewhere on the map where you can put a large structure or series of structures just for the sake of providing players a place to loot and kill monsters. It would be good for variety, not to mention you really have to wonder why of all the ruins scattered around the game. Maybe I can buy that your ancient advanced culture that is now extinct preferred living underground, but that doesn't explain Imperial forts, surely, Imperials were never known for their fondness for building everything underground. Or are we meant to assume that all the exterior structures crumbled into whatever random bits of stonework we see on the outside and that all that's left is the underground tunnels apparently everyone liked to build under their structures in the past? If that's the case, one has to wonder why the entrance is still perfectly intact. One can't help but feel like ruins are actually underground tunnels built for some reason or another and what we see outside is just decorations for the entrance...
DWEMER DUNGEONS ARE THE SOLUTIONS FOR OUR PROBLEMSS!!1 (I dont know why im soooo hyped up for them!!!) HUGE BRONZE TOWERS WITH PIPES ALLL THE WAAYYY
Sorry to dissappoint you, but if they're like in past games, Dwemer ruins will mostly be underground too.