He still had a general plan. Even an outline is something. And things change during filming, that happened with LOTR as well, but starting with a real plan puts you on the right foot. Also, the idea here is that the screenwriters you hire to work on your new Star Wars movie are better than George Lucas. There is no excuse for not planning these movies out. It's a mind-boggling decision.
How'd he have a general plan when Vader actually being Luke's father is central to the entire redemption arc of Vader and the Emperor was there for story purposes as the wedge between Luke and Vader? He was originally a separate character from Anakin, and Vader originally betrayed and murdered Anakin before actually being the same person and Obi-Wan technically told the truth via point-of-view.
It's also nearly impossible for three artists to have the same vision for a trilogy of movies or video games. That's asking a lot. You'd have to basically have a committee to do it, or you give it to one person who maybe wouldn't change his mind.
Lucas had no plan. You can tell based on how the films turned out. Luke kisses Leia or straight up smooches her in two movies but they're brother and sister. The Death Star was destroyed -- a system that we would find out started construction in Episode 3 and took a couple decades to hammer out -- but was quickly rebuilt and even more dangerous than the first. Vader killed Anakin but actually was Anakin.
He didn't have a real plan because hell if Lucas knew if he'd get a second shot. If you like the OT, you have to accept that there wasn't actually a grand plan for it, and Lucas' statements that he actually did don't stand up to scrutiny and his own treatments that he wrote of these movies.
This should have been Rian's trilogy from the start.
I really can't wait for his and for what he's going to do with it.