The second biggest rumor of a Star Wars spinoff film is actually the one that has been rumored to come next.
The regular rumors regarding this project would certainly indicate that there’s some validity to it and Lucasfilm certainly needs to be actively working on something if it plans to release a Star Wars film in 2020, but we don’t know for sure that this is what the studio is doing.
A movie about Obi-Wan Kenobi that is set between the original and prequel trilogies has been consistently rumored for the last year or so, including word that the project is in active pre-production as we speak and is planning to come out in 2020.