* More underwater features, tunnels, etc
* A small town like rorikstead with a buy-able house
* More morrowind-esque landmass, or more places like that around markarth where there's low lying fog/clouds, rocky hills, blight and moss covered trees.
* More clothes (for mages) with imbued effects
* Extended Faction quest lines
* Remove fast travel to places that aren't major cities as it makes horses pointless and makes immersion feel more like de-mersion
* Children from other races
* Partial Morrowind expanse, or at least a quest that takes you to an area within morrowind (like in a dream sequence that contains you to a particular space)
* A quest that takes you forward/backward through time in tamriel/skyrim, so you can revisit a particular space during a different time (seeing winterhold before the collapse or something similar would be simply epic!)
* A quest to aid the new emperor to power
* More Thalmor involvement
* More race specific things (quests that only argonians can undertake, thalmor that only high elves may assist, etc)
* More verticality (like markarth city's many leveled structure)