First: we do not know what the Skyrim DLC will be. I'm not pretending otherwise. But Todd Howard has been musing upon the nature of add-on chunks for the game of swords and shouting, and while he's obviously PRing to some degree, it sounds like the thinking is along ambitious rather than cyncial lines. Bethesda are looking at "ways to make the game better, not just have more, because the game is so big," Howard told Joystiq. "So we're going through ideas right now, and processing everything people are doing in the game, and trying to think of ways that we can improve it." I would take fixes and thoughtful improvements to abilities and world behaviour over another chunk of quest-littered terrain any day.
That's all we know, other than that "they won't be quick" releases. Which is also good news - I don't want meagre, buggy stuff. Well, buggy is all but a given going on Bethesda's track record, but let's hope for "only a bit buggy, eh?"
It's speculation/request time, then. I'd particularly like an expansion of the thieves' guild stuff, so I'm not just doing petty crimes after having mastered all that. And also the ability to dress NPCs up in other people's clothes instead of simply nudifying them. I won't be happy until everyone in Whiterun is wearing Falmer armour.
Oh, and Howard also commented a) that the game had sold much better than they'd expected and b) yes, they're aware of the now legendary bucket trick (see below, in case you somehow missed it), and there's been an internal argument over whether to patch it out. Embarrassed programmers say yes, but amused designers say it's "our favourite", basically. Sounds like it's staying, thank Nocturnal.