Perhaps a re-visit to Solstheim, but I'd personally like to see Atmora.
yh I'd second this.
Atmora definitely intrigues me.
Solsteim will be great as it is now a major Dunmer settlement so this'll be interesting to see the change. I'm guessing if they did then a boat from Windhelm would be the logical choice as Windhelm also has the Dunmer quarter.
I would mind seeing a bit of High Rock to feature. I'd like to see some real up-to-date Breton lore and I doubt they will be in any rush to make a full game featuring in High Rock as we already have Daggerfall. We have the Folsworn in Skyrim who give us a glimpse of Breton culture though they are a unique sect of witchmen. As Bretons are mostly influenced by the Celts, I'm guessing something like Gandalf type wizards / druids alongside tartan / stripe or fur wearing warriors living in stone houses covered in mud and grass
wizard http://1.bp.blogspot.com/-N8LPUtEuMMc/TckiagYl6UI/AAAAAAAAF0g/yVsDWljgdYs/s1600/Magic_Wizard.jpg
warrior http://eshop.friendshipmodels.org.uk/WebRoot/BT2/Shops/BT3667/4B8F/96E7/C28B/3579/91B5/0A0A/33D4/9F08/WF12003.jpg
house http://www.woodlands-junior.kent.sch.uk/homework/celts/images/house.jpg.
I would like to go to Summerset Isles to fight the Thalmor.
I reckon you will but in the next TES and not in an expansion. It could even be the Aldmeri Dominion which would include both Summerset Isles and Valenwood making it the biggest map yet. Expansions are usually centred on one region / race.