Morrowind had (and still has) a huge modding scene. Especially because it's even more mod friendly as its successor. A big plus back in the day was that the Construction Kit came with the original game disks. So you had direct access to the real dev tools. Like Unreal came with UnrealEd but with access to an Open World RPG instead of "just" a shooter.

Nexus Mods started as portal for Morrowind mods.