For my own observations:
Did anyone notice the boat sailing in 1st person at the very beginning of the trailer? Sort of looks like we might be able to sail boats?
Also, the dunmer architecture. If my mind serves me right, there wasn't really any of it in Solstheim. It was all in Morrowind. Not to mention, there weren't any mushroom-trees in Solstheim either. Now, the island of Vvardenfell was destroyed by the eruption of Red Mountain. So that possibility is kind of out I think. I don't think they would open the entire mainland of Morrowind or even a sliver. Now, I remember the islands north of Morrowind, which may be a descent possibility.
Also, did anyone notice somebody riding a big warthog-like creature? And, looking at it a second time, I realized they were wielding spears. Interesting.
Though it is true there was no dunmer architecture on solthsiem in morrowind there is now.
In TES Lore it says that when red mountain erupted, and the argonians attacked morrowind, the dark elves were forced to flee to solthsiem, where they mainly live now. If that were the case, then in 200 years was ample time for them to grow telvanni houses and build skar crab style houses as well.
(in morrowind towns like Balmora and Seyda Need have only been around for like 15 years max. Vvardenfell didnt have any major settlements until around 3E 412 or something like that - because up until then all of vvardenfell district was holy land that belonged to the temple - and Morrowind takes place in 3E 427. so theres more then enough time to build full towns in 200 years)
Also what you saw riding a warthog thing is called a Reikling. They were enemies in the bloodmoon expansion that lived on solthsiem (and there was a book back then that claimed the reiklings were the remnants of the snow elves - which has now been proven to be false with skyrim) and they had lances in bloodmoon, so its cool how they gave them spears again.