Well, let's see where each was set:
- TES I: Arena - Entirety of Tamriel (no wilderness between towns, though)
- TES II: Daggerfall - Illiac Bay (northern Hammerfell + southern High Rock)
- TES L: Battlespire - Oblivion
- TES A: Redguard - Stros Mkai (part of Hammerfell)
- TES III: Morrowind - Vvardenfel (part of Morrowind)
- TES IV: Oblivion - Cyrodiil + Oblivion
- TES V: Skyrim - Skyrim
Now, in the past, it's known that preliminary (pre-production) plans for TES III and V had called for it to be set on Summerset Isle. (the original name for TES III was going to be "Tribunal" and set there, but it was changed by 1998 or so to "Morrowind," and the name re-used for an expansion to it) I don't recall any other specific competitors to IV, but also apparently a lot had been suggested for Valenwood for TES V.
Overall, I'd PREDICT that Summerset would likely win out for TES VI, given that it's been given the backburner at least twice. Meanwhile, once we count in the official games, expansions, the spin-offs, as well as the
Elder Scrolls Travels games, we've seen each province in this many games:
- Skyrim - 5 games (I, III, V, T1 & T3)
- Hammerfell - 4 games (I, II, A & T3)
- High Rock - 3 games (I, II & T3)
- Black Marsh - 2 games (I & T2)
- Cyrodiil - 2 games (I & IV)
- Morrowind - 2 games (I & III)
- Elseweyr - 1 game (I)
- Summerset Isle - 1 game (I)
- Valenwood - 1 game (I)