A lot of what I see mentioned would more properly be a whole new game rather than DLC for this game. If the other provinces in Tamriel are as big as Skyrim I would hate to see them relegated to just a DLC. Other things, like village building seem more like the things I have seen modders do.
How about taking the really old versions of the game, Arena, Daggerfall and Battlespire, and re-building them on this new gaming engine? No need for re-scripting very much just designing the elements and adding them to Skyrim.
Expanding the faction quests seems like a natural. All the factions. Plus something a little deeper than chasing skeevers out of people's homes or tracking down overdue library books. I love the idea of creating a Foresworn faction. Perhaps even create a Thalmor faction, they recruit local spies. The Bards college could be changed into something worthwhile, this would only work if they could add more music into the game, and give music a few magic-like area effects. A whole series of quests could be built around a Khajiit faction, too...either helping the caravans or investigating their gypsy-like crimes.
More things. Weapons, armor, just design variations. More ownable homes in more places. This is the stuff modders do, but I'm on a PS3.
Along with Nameless1 I would like to see better development of the NPCs. Since this would require more dialog and writing it would probably be better as a DLC than as a mod. They should evolve more as the passage of time and events dictate. The guards currently do some of this..."so your the killer of the Glen Morril Witches"..."Kodlak is in a better place now" At least the guards are reading the newspapers.
A little more situational awareness for NPCs, too. They notice if I'm walking around naked, or if I look sick, but they should notice if I'm walking around in rare or unusual clothing and armor. Again, sometimes the guards notice, but not the general population. If I haven't come back to a town for a whole month you would think someone would say "Where have you been?"
Seduction. OK, in what I line of fantasy fiction do heroes need to get married before they get a little "Lover's Comfort"? C'mon. Like that =SPOILER= spy from Cyrodiil in Marketh. You know darn well that, if you saved her, she would be hauling you up to her hotel room after a little questioning=END SPOILER=. Or that slutty barmaid in Falkreath. No need for finishing move cut-scenes here, they can just fade out when you reach the bed. Its about character development. Perhaps an amulet of Dibella would work when your on the make in the same way an Amulet of Mara works for marriage.