The Morrowind Rebirth mod was first released in 2011, and has been regularly updated since then. The latest patch brings it to version 5.3, and as modder Trancemaster_1988 says, "with that another massive amount of changes. This time I've spent most of my time building a more believable worldspace, so you can expect to see a LOT of changes in that department."
A major effect of Morrowind Rebirth is to alter the layout of the settlements and landscape. The island of Vvardenfell feels alien the first time you visit, but that feeling fades over time. Morrowind Rebirth brings it back, and the 5.3 patch continues tinkering in that vein. For instance, in Seyda Neen—the first settlement you visit—the lighthouse now stands on a separate island rather than a pile of rock beside the town, and has been elevated further. The town of Suran has received a major overhaul, a system of wooden bridges has been added to the ashland between Seyda Neen and Hla Oad, and the Ghostgate's gained an expansion "including a new guardtower, shelter for Temple pilgrims, and a small Inn."
There's plenty more as well, including more weapons and artifacts, and tweaks like shopkeepers no longer attacking if you steal something of little value—unless you repeat the infringement. Candles now have glow-maps too, which give the illusion of wax being transparent, one of the many tiny changes only those who've spent hundreds of hours playing Morrowind are likely to notice.
Morrowind Rebirth is a wonderful mod for those of us who need to make the classic RPG feel unfamiliar again, and it can be downloaded from ModDB and NexusMods. If you're new to Morrowind you're better off sticking with the vanilla version, but if you absolutely have to start with some mods I'd recommend the Morrowind Graphics Extender and Better Heads, with the Code Patch to fix some bugs.