Absolutely the Summerset Isles.
Why? So many reasons.
Remember how interesting Morrowind's world was? Giant mushrooms, barren volcanic fields, that sort of thing? In Summerset, there is so many more interesting places than in Oblivion or Skyrim! How about a city carved into coral? Bethesda could even add Pyandonea, the mysterious jungle island south of Summerset. Little is know of Pyandonea, so it could lead to some interesting quests and creatures, including the Maormer; the Sea Elves.
The Psijic Order come from Summerset, so maybe they could have a quest line of some sort?
Here's a few cut's from the Lore page, and my comments.
''The island has idyllic orchards and clear pastures, still and silent lagoons, misty woodlands, and the unique Psijic architecture that seems to be as natural as its surroundings. The Ceporah Tower is a relic from a civilization that predates the High Elves by several hundred years. It is the location of the Dreaming Cavern, which Sotha Sil once used as some sort of portal into the realms of Oblivion, but which has since been sealed off by the Psijics.''
See? Lagoons, woodland, pastures, and cool new architecture.
A portal to Oblivion? Maybe a quest line for the Psijic Order?
''The island was once removed from the world in a similar fashion to Umbriel, but reappeared about 500 years later. The island once again disappeared around 100 years before the Skyrim civil war, and has not been seen since.''
This is perfect! Maybe you start off in mainland Tamriel, and the Thalmor are being exiled and forced to board their ships and flee, without any idea where they are going. If you choose to play as an Altmer then you can be a Thalmor officer, and if you play something else you are a prisoner! Maybe, for some reason, your ship strays from the others, and you rediscover Summerset?
I'm excited now! DAMN IT BETHESDA, MAKE IT IN SUMMERSET!!
