You have at least three entire playthroughs-worth of content to go through with Tamriel Rebuilt. It adds the rest of the entirety of mainland Morrowind for you to explore, which is about twice as much land area than base game Vvardenfell. There are tons of new towns and cities, quests, NPCs, locations, creatures, etc. in the mod.

And there are “sister” projects like Skyrim: Home of The Nords and Project Cyrodiil, whose aims are to rebuild the entirety of those provinces accurately based on lore from the time Morrowind came out, within Morrowind itself.