Underground old cities - Like old mournhodl with crumbling rocks and lots of vines and creepers on the buildings
More Darkbrotherhood sanctuaries - Like how the morag tong had like 10 guilds in morrowind, all which were really effing kewl. They even said there were many guilds in cyrodill.
Bandit Camps - not a single tent like a huge camp with a wooden wall and multiple tents, and the ring leader could have his big tent with lots of loot.
Monastaries - Only one was really seen outside the chorrol, it would be cool to find these monastaries in remote locations as reststops for wayward pilgrims.
Mousuleums and Graveyards - Outside the town
More interesting sewers like tribunual, i tried exploring the imperial city sewers and they were pretty effing boring
Haunted Towns
Drug Plantations
For morrowind it would be cool to have huge burial mounds, like an island with multiple interconnecting tombs and maybe an undercity like the burial mound but with traders and stuff.