Right now, I can see three major areas on the map with tons of unexplored territory in them:
1) Elseweyr, Southern Cyrodiil, and most of Black Marsh.
2) The rest of Skyrim + Solstheim.
3) The rest of Hammerfell and Western Cyrodiil.
Personally, I'd like to see more of Skyrim come back, especially Whiterun and the Throat of the World. We could even see Winterhold before most of it collapsed. As for what the story could be about, I'm thinking it would have to be something dragon-related. Even though the dragons aren't due back for another couple thousand years, it would be a huge shame to get to explore that hugeass mountain again and NOT have it be related to dragons in some way. There could even be a Skill Line of Dragon Shouts (Aura Whisper to reveal stealthed foes, Unrelenting Force to knock people off balance, Whirlwind sprint to be an alternative to Retreating Maneuver, etc.).
Elseweyr is the province we've seen the least of so far, so I wouldn't be disappointed if we end up going there instead. Not sure what the campaign could be about, but the scenery would certainly be nice. As for Black Marsh, something involving the Hist would be appropriate, also something exploring the enslavement of the Argonians by the Dunmer.
As for Hammerfell and Western Cyrodiil...I'm not sure. It would be nice to have something about the Adamantine Tower, given how we can all see it from three separate zones yet none of us can visit it. It being the oldest structure on the planet, I'm very eager to see if there's anything cool inside we can take.
Now for places outside of Tamriel. There's The rest of Nirn, there's Oblivion, and there's Aetherius.
Akavir would be an interesting place to visit, and it would give ZOS an excuse to add new playable races to the game. I also don't think it's ever been visited in any game so far, or even in any other forms of Elder Scrolls canon. It means they get to be much more creative with it than the other locations. Less restrictions. Atmora might also be cool for similar reasons.
Oblivion could also be interesting. There are realms we've visited in past games like the Deadlands and the Shivering Isles, and there are also realms we've seen nothing of, like Jyggalag's realm (would be interesting to see it, even if he's not there to rule over it), or even Sanguine's various pocket realms of infinite wine and prostitutes (that one visit in Skyrim doesn't count as you're only there for about 2 minutes).
Aetherius could also be cool as we don't see a lot of it.
Guildmaster of Spectral Liberty - Xbox One - European Megaserver