We would need to solve space travel. Seamless travel from planet surface to space and back would make or break the project imo. I believe it can be done and have ideas on how to go about it, but I'm not a programmer and will leave that to more knowledgeable members.
The biggest time sink would be world building. Tamriel Rebuilt has been going on for almost 20 years and that is just for Morrowind. Building multiple worlds with civilizations, nations, cities, towns and communities would take a long time unless we can use AI to help generate them.
But I'm crazy enough to give it a try. But we would need a lot of help.