Honestly, I really can't imagine a whole game set in just Valenwood. It lacks a certain... something. Maybe it's the cannibalism, or the Green Pact, it just seems that a sequel to Skyrim would need something more. Elsweyr is a hard sell as well. Culturally, it is a much better candidate than Valenwood, but then again, are tha Khajiit something you can really market well? I would combine the two, not only to present a better experience, expand the world and present the two cultures, but also the relationship betweent the two races and cultures.
You have the Bosmer, one of the most unique cultures in all of Tamriel, a race of elves forbidden to harm the forest, or harvest any of its resources. They relly on imported wood for construction and crafting, survive on a diet of meat and cheese and create weapons, armor and clothes mainyl from animals. Then you have the Khajiit, two very unique cultures, a warrior culture, shaped by the harsh conditions of Anequina and a jungle culture, formed around the persuasive power of coin.
These two cultures, not only are they both lacking in some ways in order to present a more complete Elder Scrolls game, but they also bring something unique to the table. I would combine them, as the Thalmor unite them under a common banner. The historic rivalry between the two would be on full display, as would their cultural traits and traditions. The story would foucs on a conflict between them, dark forces which brough them into conflict and several figures, united by a hero to save both from mutual destruction.