But I can't help to think that from a marketing point of view it is better to not make a game set in a province that already exists in another game.
If TESO gives us all of Tamriel (judging from the announcement it will), do you really think TES VI will be set in one of the provinces that already exists in TESO? I personally hope the MMO and single player games will be truly separate, but the marketing team of BGS could very well have a different view. TES fans do not mind a province that has already been done, especially because it's a completely different time frame. But your average Joe when hearing about TES VI: Hammerfell (random example) might think "I already know that from TESO". It's bad from a marketing point of view.
I wouldn't be surprised if TES VI will not feature one of the provinces of Tamriel, but something different. Or that TES VI won't come before 2020 at which point TESO is pretty outdated.