I could see them doing a single-player game that spans all of Tamriel. But I cannot see them integrating that game with ESO in any way.
TES:O is actually my idea of exploring Tamriel, I love playing it on the PS4.
http://www.elderscrolls.com/arena/, technically speaking. If they're going to make another large-scale Elder Scrolls game, I think they should do like Daggerfall and limit it to one or two provinces; just have the provinces of more realistic scale. Of course, that's assuming they could do it without losing the level of detail present in recent titles, which seems unlikely.
I would like to see a Elder Scrolls with all of Tamriel, but only if each province had as much detail as each individual province did in the previous games.
The only way I can see that happening with any semblance of detail would be for Bethesda to make each game "modular". Each new game would add onto the existing landmass of the previous game, so your character could visit previously released provinces as well as the new one. Each new game could also update a few items of the previous games' graphics, so things wouldn't get too far behind. Since a lot of items are used in multiple games, and a lot of equipment is common to more than one province, not everything would need to be done from scratch. If Bethesda ever got done visiting every province, we'd have a "complete" Tamriel.
ESO is the closest you get, it shows you all of Tamriel, in that it at least touches all the provinces. Quests are pretty good too.
It does not have the detail level of the tes games but an single player game in all the provinces would have less details than ESO.
Not all the provinces, Elsweyr isn't in TESO, you mostly visit an Island off the coast of Summerset Isles and pretty much all of Valenwood for the Aldermeri Dominion.
http://i.imgur.com/Uwwxy0Q.jpg.
And what many OTHER gaming consumers want, is ONE province done to detail, rather than several half-assed ones.
True, reaper march is the closest. many other areas like Sumerset isles and black march are just one zone, overall perhaps 40% of the map.
Stil its the closest we get.