Plans when the construction set comes out:
A bookstore
An ultimate stealth dungeon. Maybe a dwemer ruin, maybe something else. Point is, it would be set up so you have to be stealthy, and if you fail you die. Up the damage on traps so getting hit by a poison dart actually means something (It means you're gonna die), throw in a room where you have to sneak past some kind of highly alert enemy that'll fuck you up if you fail (I'm thinking some kind of upgraded steam centurion with near-instadeath steam blast). Stuff like that. A challenge that's insurmountable for a low level rogue or a non-rogue, and seriously taxing for a rogue.
If they succeed, some kind of artifact. I'm thinking a light armor head piece, since most rogue headpieces are lack luster. My current idea is a hangman's hood (There are three of them in the room where you first meet the Dark Brotherhood. I'm deliberately avoiding spoilers) with toggleable night vision and life sense. Maybe paint an eye on the front of it.
Darker nights and dungeons. The darkness is so not dark right now that night vision is useless.
Mephisto:
This is a Morrowind spoiler, but Morrowind is ten years old so I'm not putting this in spoilers. Look away if you don't want to see it. The Dwemer disappeared when Kagrenac attempted to siphon power from the Heart of Lorkhan. What exactly happened to them is unknown. They may have been destroyed, they may have been sent to another plane, they may have apotheosized into some kind of collective consciousness. Nobody knows. The dunmer say they were punished by the gods for sciencing too much.
Tamriel mod:
There was, and presumably still is, work on a mod for Morrowind that adds the rest of Morrowind (Morrowind itself only containing the Vvardenfell province). It's using a weird development strategy where people can apply for a volunteer "job" and if they prove their content creation skills they can claim a section of the map to work on. It's very slow going and I doubt much will come of it. There's already quite a bit done and playable but it's very buggy and missing a lot of the actual meat.