I'd like to see the Bard's College fleshed out into a real faction with its own story arc. Along with that, maybe add some features that let you actually play as a bard. Musical instruments already exist in-game -- let me play them. In taverns. And earn money proportional to my speechcraft skill.
One of the problems with Skyrim DLC is that it's pretty much impossible to top Shivering Isles, where you literally become a god. I guess the Dragonborn could assume the mantle of yet another Daedric prince, but it's been done. Visiting another plane of Oblivion might be fun, though. I'd like to see Azura's realm, although it might be hard to do it justice. It's described as being unimaginably beautiful, and anything the game designers can create is, by definition, imaginable.
Another Shivering Isles-style adventure might not even be implausible, lore-wise. Sheogorath's line in Skyrim about his title being "passed down from me to me every few thousand years" might imply that the events of SI wasn't the first time a mortal assumed the mantle of Mad God, and maybe that's true of all Daedric princes. The biggest problem is, like I said above, it's already been done.
Speaking of SI, where's Jygallag? I like to see him featured as yet another Daedric prince now that he's free of his curse. Could that be possible DLC material?