I want a big expansion, and something that deals in beating down the Thalmor.

Building my own manor/fortress from the ground up (like in morrowind) would be an insta-buy for me

I'm really hoping for a High Seas type of adventure. It would be very fitting. After all the Nords are modeled after the Vikings. Imagine getting your own ship and crew and navigating all the waters around Tamriel. You could dock at island ports and go exploring! The game could even introduce water dragons that attack your ship!