Vader literally means father in Dutch, and George Lucas himself said Vader was always meant as a variant of Father before ANH was shot.
http://www.rollingstone.com/movies/...he-cult-of-darth-vader-20050602#ixzz3omEOdJNc
That may be Lucas retconning his own life but still.
George Lucas also said that American Graffiti is a remake of I Vitelloni.
George Lucas says a lot of things.
As a side note, I never understood why would he or some fans insist on this point, it's just a question of process, some writers plan everything in advance some don't, neither approach in inherently better.
There are some great writers who have no idea where the story will take them when they start writing. I guess maybe part of it comes from the desire of some people to think of these movies as a peek into this fully formed universe. I don't know, I think they're just, you know, movies.
That's not entirely true, and yet you are still half right.
Lucas did have a much larger story than in the original movie - he had a whole story bible notebook and multiple movies worth of story - but it certainly wasn't the plan that we actually saw realized in I-VI.
Lucas did know the original movie might be a one-off...And yet he did want "Episode IV - A New Hope" on the original movie, but Fox (or the common sense of the day) wouldn't let him.
Lucas didn't necessarily want the title "Episode IV" on the movie because he had concrete plans for a I-III and V-VI, but because he loved the feeling he had walking into saturday morning serials like Buck Rogers when they were already on Episode XII and he had no idea what what happening. Or the feeling of watching The Hidden Fortress and not knowing shit about Japanese history. He liked that feeling of wandering into the middle of the story knowing fuck-all about the world, and having to catch up anyway. A New Hope is definitely that kind of movie that starts in media res and doesn't quite conclude the overall story... If that's the only movie that were ever made, Lucas would have felt satisfaction that it worked on that level. And so "Episode IV" was kind of a clever nod that it was the middle part of some sprawling saga you'd never see the whole of. Very clever for 1977.
But you are right in that Lucas didn't exactly know where it was going. In the evidence we have of his original notebook "bible", it was a whole lot of random crap that didn't necessarily make it into the movies. It's a typical pie-in-the-sky notebook of random ideas.
I'll admit, I haven't dug super deep into the history of the making of Star Wars. I saw a documentary once, and was exposed to some other info through videos and articles over the years, but nothing really exhaustive.
But I've seen the film, and nothing on screen suggests that this story is anything but your standard standalone adventure with very obvious calls to serials like Flash Gordon (which is what he originally wanted to make, right?),
Maybe it was all studio execs meddling, I didn't get the sense that it happened to a large degree, but again, not an expert, but if it was, I think it was for the best. And I know it's not a really popular opinion, but I kinda wish Star Wars stayed like that.
