According to the “Icons Unearthed” documentary, Lucas came up with the “Vader is Luke’s father” idea before Brackett’s draft was written, which is interesting. I assume that for whatever reason, he didn’t tell her—maybe the idea hadn’t been 100% finalized by that point? Though I haven’t actually seen the documentary (I’m just going off what I’ve heard from others), so although it’s clear that he had come up with the twist before Brackett’s script, I don’t know how finalized the idea was. I suppose it’s also possible that Brackett knew about the twist but ignored it, but it doesn’t seem likely that she’d ignore such a major plot point.

Also interesting is that the Kaminoans were basically recycled from this draft, which has similar aliens on Bespin. They’re described as having a similar appearance, they ride flying whale creatures, and IIRC their use of saber-darts (including the term itself) was an idea taken from this draft. It’s entirely possible that the Kaminoan city was inspired by Cloud City too, what with its similar appearance. It’s unclear whether all of this was Lucas’s concept, or if it was Brackett’s concept that he borrowed. However, if Lucas couldn’t be bothered to tell Brackett about the Vader father twist, it seems unlikely that this was one of Lucas’s ideas—unless this was a concept that Lucas (or McQuarrie) had come up with much earlier. So even if Lucas considered Brackett’s script flawed, assuming the Bespin aliens were her idea, he still was willing to borrow some concepts from it.