Quite agree. We already have Morrowind and Solstheim.
What I would love to see is a huge underground expansion ( not like Unearthing Mehrunes Razor, that was just a big dungeon ). Think of a huge Dwemer city, colonised in part by friendly Nords or Dunmer, or does anyone remember the old ADnD modules Descent into the depths of the Earth and Vault of the Drow. Nowhere we have seen, and as it's not on the map, no danger of overlap with future TES games.