In a recent interview for The Director’s Cut, The Last Jedi writer and director Rian Johnson described to Spike Jonze how he wrote the script for episode 8 of the Star Wars franchise. To the likely surprise of many, he was not provided with a Grand Plan for the new trilogy.
Johnson apparently started off by having a chat over a meal with The Force Awakens director J. J. Abrams. Abrams gave his view on where he thought the story should go, and that was that. Johnson then wrote a first draft of the screenplay and submitted it to the powers that be, presumably important Disney and Lucas folk such as Kathleen Kennedy.
It’s unclear how much was changed by the keepers of the Star Wars story, but this interview suggests that he had broad leeway to do as he pleased with the movie, and that there wasn’t much if any interference on the larger points.
The grand theories proposed by us fans were wrong to a large degree because the only one who really knew what was going to happen was Rian Johnson. Now that EP8 has been made and the “baton” (as he put it) goes back to J. J. Abrams for EP9, even he doesn’t know how it ends! The whole thing is a Shroedinger box for a couple more years.