One of things I would like to see would be something involved with the College of Winterhold. I was never satisfied with the College's questline and something more to it then asking the librarian where I could find a Moth Priest would be more interesting.
Exploring new realms of Oblivion would be neat to see as well. In the Elder Scrolls series I've been in the Deadlands of Mehrunes Dagon, and the Soul Cairn (though I'm sure there have been others in the series since all I've played is Skyrim and Oblivion). If there have been other appearances of other planes then I would like to revisit those at some point.
A continuation with the Civil War would be nice. I didn't feel that the war just simply ended with all of those Stormcloak/Imperial camps left all over the place. Maybe a siege would be a cool thing to do, working a siege engine or storming the walls via ladders and towers would be so cool to see.
I might've missed something cooler in previous posts, but either way, those are somethings I would like to see.
Hear, hear!
Some other ideas:
1. Upon completion of the civil war, the winning faction should give the DB the assignment of 'cleaning up the house.' Every single bandit lair, vamp lair, witche's lair and necromancer's lair needs to be CLEANSED. All giants need to be STOMPED. Anyone and anything that interferes with the regular flow of people and commerce along the roads and major paths of Skyrim needs to be permanently dealt with, including Labyrinthian.
2. By definition, #1 above includes dealing a 'final solution' on the Forsworn.
3. There may also need to be a campaign to suppress the falmer, who are clearly starting to raid on the surface and infiltrate surface structures from below.
4. The Thalmor will most assuredly interfere, as a weak, hobbled and divided skyrim is to their advantage. Thus, the thalmor will have a major role to play in this 'sterilization' campaign and will need to be dealt with effectively.
5. Perhaps one of the daedra might serve as the center of an alternate 'faction' that wants to disrupt or prevent this 'sterilization' campaign. Could make for a fascinating twist.
6. I wonder what the dragons would have to say about all this? Another interesting twist...........
The above would provide perhaps 50-100 hours of new content on the existing map. That would be very, very cool, and less of a development problem for Bethesda than introducing additions to the map.