1. An expansion could explain the disappearance of the Dwemer, maybe have a few survivors appear.
2. Werebears. I know that Werewolves are already in Skyrim, but Werebears are supposed to be the most common Lycanthropes in the land of Skyrim. I'm not sure what differences they'd have. Maybe they'd be more like the Werewolves of Morrowind and must feed on an NPC once a month or else lose health. Or they could just be something to kill, or a maybe new faction to join (similar to the Vampire Lords of Dawnguard). I've also wanted to see a Wereshark since I read that there are rumors of their existence in Tamriel. They would definitely be different.
3. It would be nice to travel outside of Skyrim, especially to the lands of Elsweyr or Black Marsh. These would be nice for exploration, since no game since Arena have gone there. But Cyrodiil might be a better option, with all of the political struggles centering on what happened there 200 years ago. This could also be a good way to reintroduce the Fighters and Mages Guilds, offering new quests and such. With these new lands, new houses could be made available for purchase. Also, we could possibly finally see what the full Colovian Fur Armor looks like, along with the hat M'aiq has been missing.
4. Sort of similar to 3., but not really. It might be cool to get to travel to different planes of Oblivion. I'm still a little bored of Mehrunes Dagon's Deadlands from ESIV (same thing over and over), but some of the other realms could be interesting to visit. These other Deadra are supposed to have their own varieties of lesser Daedra too, right? So far, most of the Deadra we've seen are Mehrunes Dagon's servants.
5. Before the release of the game, it was said that a feature that was being worked on was a system that allowed spells to be mixed together to form new ones. If this could be implemented into a later expansion, it would be cool.
6. We can now fight with one-handed weapons and bows while on horseback, but mages are kind of vulnerable. I think it would be nice if mages could launch missile attacks (fireball, etc) while on horseback. Also no reason we shouldn't be able to cast healing magic. Heck, it would also be cool if we could cast Invisibility and make ourselves and our horses turn invisible. Lastly, I would like to be able to purchase a horse for my partners and ask them to ride on them. It's a little silly that while we've riding away on our horses, our partners are chasing after us on foot.
7. While this may have seemed like a redundant spell for Skyrim, I would still like to see Chameleon return. This allows a player to become harder to see while still being able to interact with objects and NPC's. Your level of skill (and possibly some new perks) determines how well the spell makes you blend in.
8. It might be cool if horse armor returned, but only if it didn't look ridiculous. Perhaps it should be limited to iron and steel. Elven armor just looks silly on a horse.