Well, there’s plenty of room for backstory, as the TV series intends to cover. But the trailer looks like it’s covering getting Jake to getting to the Tower - not sure what else another movie would be able to do. It would be weird to shoehorn Calla or Blaine in a separate movie.
Fun trivia: I watched the movie The Mist yesterday for the first time in years and the movie opens with the main charter who’s a movie poster artist doing a poster for The Dark Tower. It has Roland, the Tower and the Rose but no title.
Nice catch, Quimby! Darabont does such an amazing job with King’s work. I wish he was connected to this project…