I don't think we're going to Morrowind. Solsthiem is where some Dark elves fled after the argonians so they probably just set up shop. I think.
Solsthiem didn't have the mushroom forests, or the cities made of chitin. I would also include Telvanni towers, but those can be built/grown elsewhere I guess. Pretty sure it's both Morrowind and Solsthiem.
When the dark elves piled into Solsthiem I'm sure they built their own stuff, and over the course of a few hundred years since we last seen Solsthiem I'm sure they could have built a lot of stuff.
The part in the trailer at 0.28 where you can see the Velothi type structures you can tell they are inside castle walls, most likely Fort Frostmoth.
I just really don't see them going to Morrowind. Not to mention Morrowind is actually the mainland around the island of Vvardenfell, the place where the game Morrowind actually took place.
"Real glory springs from the silent conquest of ourselves."
Anyone else notice right when it says "and he seeks to return" it shows the player character read some book, then get magically chained up (or atleast that is what it appears to be) maybe the old dragonborn needs to absorb the new dragonborn to return?