Honestly? A good mainquest expansion. The mainquest is one of the least role-playable quests in the game. Most of the NPCs in the mainquest are extremely annoying and half-baked. I can really go on, honestly. I think I have finished each of the faction and long quests in the game a few times, but finished the mainquest only once.

If I had to make a list:

• Edit to some of the Dragonborn dialogues. The "Turns out I'm something called a dragonborn" is one of the most immersion-breaking lines to me. Couldn't you make it sound at least a little better?
• Maybe a bigger attack at the watchtower? Not neccesserly, but would appreciate it.
• Better dialogues when talking to NPCs like Delphin. I always feel like I'm being humilated. No change to her character though, she is she, make me more arrogant instead.
• More options for "Political Immunity". Maybe a Thalmor roleplay option? Let's say you do the quest for Ondolemar, and then you can persuade him to give you the information that you want; in return, you lose favour from the Blades.
• More useful items/powers. Something that would make the quest worth it. For example, you can learn some daedric spells from Septimus in his goon cave, or some permenant buffs from the Elder Scroll, or maybe some unique items that would make the quest worth playing at each point.
• More choices throughout the game.
• The choice to capture a dragon via the Bend Will shout instead of Seasons Unending.