Making expansions for existing characters is somewhat problematic. You can't really do much power progression for existing characters - they've already been there, done that, and bought the creepy mask. Creating new "addon storylines" for existing characters will inevitably be either throwaway (Here's twenty new dungeons and a major boss who you could beat with your eyes shut because you already beat the game hollow - hope you weren't already bored!) or exponential (Remember all those major world-destroying calamities you prevented? Well, this evil Imperial sorcerer is MORE DANGEROUS THAN ALDUIN!). The only real alternative is going different, not bigger - do something like Shivering Isles, where the story, setting, and challenges are so different that your world-ending powers don't really unbalance the setting.
But all that aside, what I'd like to see is one of three things:
1. Entirely new gameplay elements. Let me be a trader, a cook, or a game trapper. Let me LIVE in Skyrim. This kind of expansion would revitalize the basic gameplay on a second playthrough, giving players a fresh experience in content that might have grown stale; or let them have a "non-stressful yet rewarding experience" living in a virtual world.
2. Entirely new regions with minor quests. Let me wander off into Hammerfell or Morrowind and solve the problems of random villages. Nothing more substantial than the guild quests, please - I don't need another world-spanning plotline. Just let me be an adventurer! Heck, make a new "adventurer start" which strips out the Dragonborn storyline - someone else is Dovakhiin; you're just a random adventurer! Have fun!
3. Alternate resolutions for the guild quests. Let me join the Silver Hand and wipe out the Companions. Create a questline that works in parallel but counter to the Thieves' Guild. Basically, do for every guild what the Civil War storyline does for the Imperials and Stormcloaks - allow us to actively choose sides.
Any of these would be brilliant ways to expand on the game, especially for additional playthroughs. Admittedly, though, #2 is the most likely, least "unusual" option.