I'm running the Tamriel Rebuilt mod.
As you can see in the map in the upper right of the picture above, I am not in Vvardenfell nor Solstheim, I'm on the mainland of Tamriel, specifically the city of Andothren.
What this mod aims to do is restore the original vision of TES III: Morrowind, which was the entirety of Morrowind, not just Vvardenfell (and later Solstheim in the Bloodmoon expansion). The game wound up getting scaled down, as games often are, and this mod tries to scale it back up.
The project has been in development for nearly 20 years and is still in development today. They currently have roughly half of mainland Morrowind implemented, which is still A LOT and quite impressive for people working in their spare time. There are also other mods in development that implement other provinces into TES III: Morrowind, but I'm not running any of those. In theory, the entirety of Tamriel will one day be playable in this game... One day in 2092.
If you are wondering how you reach the mainland, they've added a few Travel NPCs to existing locations. A couple to docks, letting you travel by boat, and a couple to Mage Guilds who will teleport you. Below is the dock just north of Vivec. In the vanilla game there would only be one NPC there:
I'm not sure if this mod has restored House Indoril and/or House Dres. In the vanilla game a major plot point is the sixth house, yet there are only three great houses in the game, which may have you wondering about houses four and five. As you may have guessed already, those are Indoril and Dres, and they were originally going to be joinable, have quests, and allow you to rise through the ranks, just like Hlaalu, Redoran, and Telvanni. Like mainland Morrowind, they were a victim of the scaling down. I doubt they're implemented, I would think that would be prominently advertised in the mod description, but who knows? There are supposed to be hundreds of new quests and items, though. I got a quest already when I got robbed of 500 gold when sleeping at an inn in Andothren. After a bit of digging, I found out that it was a well known petty thief and I'm going to track her down, but not until after I fully explore the city (she's known to hide out in nearby Dwemer ruins). Anyway, I'm impressed so far and I'll report back as to the quality of the writing, art, quests, etc.
God bless these people still working on mods for a 22 year old game.