Well, I wanted Skyrim, but we're getting that. My second choice is Morrowind. We haven't seen it all yet. But then again,
So I'll go for my third choice, Hammerfell. We've yet to see all of it.
Fourth choice if High Rock. Again, we haven't seen it all. And we saw it in Daggerfall, which was randomly generated, so I don't think that counts. And it shouldn't count either, because if people don't include Arena when it comes to provinces they haven't seen, why include Daggerfall? It was randomly generated too. I'm not even sure they should include Morrowind, we only saw Vvardenfell, it's kind of like saying we've seen Skyrim already because of Dawnstar (the mobile game) or we've seen Hammerfell because of Redguard.
We haven't seen all of Morrowind, but the Tamriel Rebuilt Project is doing a decent job of finishing it, and might even be done before TES VIII releases. Besides, the province is different after the cataclysmic events following the Oblivion Crisis, and the hundreds of years since.
We haven't seen all of Cyrodiil, although the tiny sliver of land around the Imperial City was supposed to represent the entire vast province. The size of the gameworld was about right for just the Imperial Island, not the whole thing. We certainly didn't see any of the legendary Cyrodiilic subterfuge, power-mongering, and political backstabbing beneath the surface, or more than a glimpse of the rich merchant trade, agriculture, and industry that's supposed to support the vast Empire.
Dagerfall was a "stylized" look at High Rock, or at least a major part of it, but the procedurally generated landscapes really didn't do it justice.
Arena was supposed to cover all of Tamriel, but did so in a "primitive" manner that's been superceded or ignored by every game so far.
In short, Bethesda could reasonably justify going back to any of the previous provinces, but I'd rather see them tackle something we haven't experienced yet.