I think Hammerfell, in terms of possible DLC, offers the most options. Stros M'kai and the Chain would likely be included in the base game. To the west we have the islands of Herne and Cespar, as well as Cybiades, which seems to have been moved further west by ESO, and the island of Betony, located south of Daggerfall. We also of course have Stirk to the east of Stros M'kai, and Thras to the far west of Stros M'kai. Those two could constitute a fine DLC, adding a strange, alien place in the form of the Coral Kingdoms, and the somewhat familiar Colovian settlement of Stirk.
I have a couple of ideas I'm thinking over regarding Hammerfell. One idea I quite like is the resolution to the Skyrim Civil War, or as it will likely be known as: "The Stormcloak Rebellion". Basically, every major victory only bred martyrdom and escalation, and the war dragged on for years, resulting in nothing but breaking the back of the Empire and rauaging the countryside, this, coupled with a few harsh winters, has caused widespread famine, forcing many Nords to seek a better life in High Rock's Western Reach, in the Nordic city in Jehanna, and in Hammerfell Dragonstar.
I'm probably going to write-up a setting concept for Hammerfell in the same manner as I did for Black Marsha a while back, to try and paint a better picture of what the province might look like circa 4E 201. Under the rule of the Second Empire, Hammerfell was a republic. That makes me wonder if there could be any city-state republics in Hammerfell. A large part of Hammerfell's political questlines could heavily involve trade and economics, mostly dealing with its secession, as well as old grudges resurfacing in times of peace. Would Sentinel even remain as the capital city?