One problem with long-term planning in TV/movies is actors leaving/dying.
Just this week Supergirl ended their season on two of the characters getting engaged, only for one of the actors to decide days later that she's leaving the show. If they'd known that a few months ago, they wouldn't have set up an engagement. Lost apparently had big plans for Mr. Eko, but the actor wanted to leave. David Chase planned out a whole season of Sopranos about Tony versus his mother, and had to completely scrap it when Nancy Marchand died.
Star Wars obviously had plans for Leia going forward, but they've had to adjust after Carrie Fisher's death. Back in the prequel trilogy, Terence Stamp was supposed to play a major role, but hated doing the first movie and was scrapped.