1. Blackreach- would have loved more information on the slaves/servants and maybe side quests/merchants etc(resistance group or something)place is massive but not as much to do as there could have been.
2. Vale in dawnguard- more monsters, uses for vale skins. More reasons to revisit
3. Winterhold - theres the college and 2 buildings. There could have been a player house etc
4 morthal, rorikstead and other small towns and inns. For most there's 1 real quest and no real reason to revisit