Nit/Correction; Tamriel Rebuilt is not about adding all the Tamriel but only Morrowind mainland. What you might be thinking about is Project Tamriel [0] which tries to add Cyrodill, Skyrim, High Rock and Hammerfell. They're working in collaboration with Tamriel Rebuilt so they are all compatible and share many resources.

[0] https://www.project-tamriel.com/welcome