Several possibilities abound here, and it is all pure speculation on my part, but if TESV is not in Skyrim, or not restricted to Skyrim, here is where I think it could be...
Summurset Isle seems like an easy choice. This is a set of islands isolated from mainland Tamriel that would make for an easy game world. However, this land is also a possible setting for an expansion, similar to the Shivering Isles.
The whole mainland Tamriel is possible, in my opinion. All provinces except for Summurset. This would be a massive world to explore, but I think it is doable as the foundations for each province had their start in each realm of Cyrodiil. I imagine Hammerfell having architecture similar to Anvil, with more Colovian/Kvatch and Corrol style architecture to the east, and more Nord style with the border of Skyrim. Desert landscapes, mountains, canyons, etc...
High Rock would be different, but not too much so.
Morrowind would be a much different place than what we were used to in TESIII, for one because of the events in The Infernal City. It may also no longer be Morrowind, but rather an extension of Argonia (if what I am hearing is correct from the book - I didn't read it yet) - and of course Argonia itself.
Elsweyr... the work Iliana did on the Deserts of Anequina is absolutely stunning. It would be difficult to imagine the devs at Bethesda didn't take notice.
Valenwood, this province would be much different from the others. If the whole of Tamriel were not represented, and TESV was not in Skyrim or Summurset Isle, then Valenwood, or perhaps Valenwood and Elsweyr, or even the whole stretch of southern Tamriel, might be possible. Valenwood itself would be quite a unique realm to explore, different from both Oblivion and Morrowind in environment. Maybe in the woods between Skingrad and Kvatch, and the West Weald, there might be some parallels, but I'd imagine Bosmer architecture to be vastly different from anything found in Cyrodiil. I'd imagine a heavy use of natural surroundings - tree houses and such - with an overall design closer to the mushroom cities of Morrowind than the European style towns of Corrol, Cheydinhal and Skingrad. Ayleid ruins may be possible, or old Bosmer ruins as well. Would there be Imperial forts?
If the whole of Tamriel is recreated, I'd imagine the province of Cyrodiil would be similar to but different from TESIV. For one, Kvatch would be rebuilt. If the Empire is under collapse, The Imperial City would not be a very friendly place to visit - especially if one weren't Imperial.
As I said, Summurset Isle is a possible expansion, but if it were included in the original version, other expansions could be anything - Pyondonaea (or however that is spelled) or another Daedric Realm (ala Shivering Isles) or something new to the Elder Scrolls Universe.