Nothing new, unfortunately… as discussed here (if you’d like to read more about it); a few people from around that post SW’77 time were guessing, spit-balling, and throwing out their theories for what could be next for ‘Star Wars II’…
• '78 interview with David Prowse [spoilers] - he reveals Vader is Luke’s father - a 2010 thread.
Does this confirm anything?
Not really, I’m afraid. I don’t think Prowse was in the loop about the story discussions that took place in late December 1975 involving Lucas, Kurtz, Foster, and Lippincott. I’m pretty sure the “we find out who Darth Vader is in the second book” ties back to the ultimately dropped back-story idea that Kurtz supposedly heard from Lucas (that Kurtz admits was never written down, only verbally discussed).