Agreed. Hammerfell, to me, seems the most interesting because it's a Desert like area. While the landscape would be generally barren on the outside, there would be tons of secrets overtaken by sand. Another one of my ideas was a lost city that has been completely buried, and deep within it are ancient texts providing insight on the past and other various treasures. Not to mention - deadly mummies

I think it's better to not tarnish the new provinces with dlc and leave them for full games. Mainland Morrowind would be perfect in my opinion, it's never been seen by us yet it can be packed with nostalgia at the same time, and something to do with the red mountain situation could make for a good storyline. Perfect dlc material in my opinion.

More quests for the factions-

My fantasys include (no spoilers, but should be obvious enough for those who know) :

A return of the Silver Hand. An actual intelligent plot to take down your faction, perhaps by exposing their secret. A Silver hand run village/base in the mountains, which you infiltrate and do quests for while working from within to take them out once and for all.

A series of quests from a certain Daedra to her agents. Heavy on the traps and puzzles. I wouldn't mind a trip in to Oblivion to retrieve some artifact. A couple of new powers as a reward. Maybe a dagger to go with my blade and bow.

A rival agency setting up shop in Skyrim. stealing your contracts. Deeper and more varied ways of taking out targets while trying to beat your rival to it.

Also! Decent vampire content.

A big endorsemant for more varied smithing options, also.

My best guesses:

DLC Expansion 1 - Orsinium and Malacath

DLC Expansion 2 - something in High Rock (visiting High Rock on an advanced engine was a wish by Todd Howard some months back)

DLC Expansion 3 - Blacklight region of Morrowind

Dlc explaining the dissapearance of the dwemer.

edit: Please leave provinces alone, they are for future games.

I totally agree with the Thalmor. To me they are very interesting and could provide a very developed quest line and story (even new dungeons, armor, and spells). Expanding on the Thalmor definitely would make a great dlc.