Originality is vastly overrated as a key aspect of artistic strength. “Yeah, but it’s new” doesn’t carry a lot of weight when what’s new isn’t made well.

However - Lucas DID basically whip up Empire Strikes Back’s story/structure more or less all by himself once Brackett was gone. Kasdan contributed heavily, yes, but by the time he came on the bones of the thing were firmly in place. But on the other hand: The most famous twist of all time, probably, and the one that basically doomed Star Wars to forever be taken way more seriously than it ever really needed to, and trained its biggest fans to expect twists and turns and huge surprises in every chapter despite the fact it’s ALWAYS been a straightforward fairy-tale other than THAT ONE TIME… that twist was basically pulled out of his backside at the last second before shooting started. It wasn’t a deeply considered, thought-out idea. It was just a hand grenade tossed at the end of the story to create one hell of a cliffhanger for the next chapter.

It was also not really the newest of ideas, either. In fact it was, even by 1978, a pretty obvious staple of soap operas and pulp fiction.