I would love a followup to skyrim, but i know a lot of people would be upset that they aren't doing something new.
Black marsh would really suck in my opinion. I don't want an entire game based in a dark swamp with a bunch of lizard people. Would take me out of immersion for sure.
I'd like a new daggerfall, or a tamriel origins type thing that would cover how civilizations rose up and populated tamriel from their native continents. Or maybe go to these other continents.
The dwemmer should stay a mystery in my opinion. Part of my love for the elderscrolls is how they use ancient ruins and peoples to create mystery. Taking away that mystery would be tricky to pull off without spoiling the dwemmers story.