Any writer can write a Star Wars movie, but they won’t be true continuations of Lucas’s core story.
True continuations of Lucas’s story have been written by other writers since Lucas started hiring them to do so in 1976. Lucas didn’t have a core story beyond the 1977 film. If he had, he wouldn’t have commissioned two authors to write the sequel. One of which was published as the official continuation of the story. The contract Foster signed when agreeing to ghost-write the novelization of Star Wars was that he also write a sequel story. Lucas had no core story, no Saga, and no Original Vision of nine/twelve/six films.