From the interviews I've read, I gather that the Skyrim DLC will fall somewhere in between Shivering Isles and The Pitt, in terms of size and scope.
Yeah, I remember that Skyrim DLCs would not be like Oblivion. They'd be more akin to Fallout in terms of scope. They probably won't add a new area every time but definitely would have a quest arc if it doesn't.
On topic:
Here's a DLC idea for you: Dovahkiin Reborn.
Dovahkiin RebornA total overhaul of the Civil War quests. I found that the Civil War was missing something really important...substance. There was almost no substance present in the quests after the Siege of Whiterun. So there should be a DLC that overhauls the entire Civil War system...and gives you something extra.
During the Civil War, instead of a linear plot, it should be dynamic and work better with the radiant feature. Since I'm for the Empire, I'll go this route: Instead of just going to Point A, killing everyone, and reporting back, how about some variety? Like you have to rescue a bunch of Legionaries or Stormcloaks are attacking a crucial point and you're being sent to help.
Maybe even have one city battle for the Legion and one city defense for the Stormcloaks to experience both sides of a city battle after Whiterun. But to keep variety and not just copy paste like Whiterun, you have to secure objectives? Perhaps take out a catapult/ballista?
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Now for that extra thing: Why not lead your own rebellion? You have the Shout, many people consider you a hero, and you may or may not be a Guild Leader. Only criteria for this is that you have to already beat the Main Quest.
But no, it won't be like an RTS. You'll lead your troops but you'll be the one charging into battle and supporting your troops in the major battles. Maybe be able to set your policies and what you stand for, which will affect who joins you.