I've seen few topics with suggestions about new TES game and expansion. But what of them is better for Bethesda, more logical and thereby have more chances to be created?
I think there could be either empire's political/diplomatic counterstrike (joining lost provinces) or victory of aldmeri dominion. In first case we will probably go to Hammerfell at first and then either to Elsveyr or to Black Marsh (it will probably be TES VI, but who knows, there is New Vegas after all). In second case it will be Summerset Isles.
From other point of viev Elsveyr is province with missing moons and there was no TES game without "epic, mystical and important"(Daggerfall had Numidium, Morrowind had heart of Lorkhan...). But there is Elsveyr mod to Oblivion (I'm not sure about details but still) and it's like Black Marsh also have something to offer (I wasn't really intrested in that province, so...). And Thalmor could put whole world at risk by simply being themselves.

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The orc kingdom of Orsinium is probably about the right size for a DLC. It borders Skyrim anyway and would be the size of a hold with a main town. That's perhaps similar in size to the Point Lookout DLC for Skyrim so it seems feasible and could be a nice exotic addition.

I don't think a DLC or expansion would cover an entire province as a province is too big and only really suited to release as a whole game, unless of course they only did a slice of one as an add-on.