Some fantastic answers here, but I thought I'd just add a little more detail, that I don't think has been included in any of the others. Firstly, as has already been said, STAR WARS was only ever one film. Vader wasn't Luke's father, Leia wasn't Luke's sister, no further films were planned.
Even the original 1977 opening crawl looked like this:
That's right. No "Episode IV : A New Hope".
It's difficult to remember now, but STAR WARS was HUGE. I mean TITANIC big. Adjusted for inflation it's actually still the second highest grossing movie of all-time (AVATAR is only number 14).
So Lucas and his friends set to work on expanding the universe, much like the Wachowski's did with THE MATRIX. Lucas's "second in command", from the beginning of his Hollywood career, was a man called Gary Kurtz. Kurtz produced Lucas's first "Hollywood" feature, AMERICAN GRAFITTI, which in itself was a big success (currently 44 on the highest grossing films of all time list), STAR WARS, and THE EMPIRE STRIKES BACK.
Lucas worked with Kurtz (and many others) on producing a rough outlines for approximately 8 more movies, making 9 in total. The EMPIRE STRIKES BACK was made according to this plan.
After the success of RAIDERS OF THE LOST ARK (the project Lucas put together with Steven Spielberg), Lucas decided that cinema-goers wanted more of a rollercoaster action movie, and so changed the 9 movie plan.
Kurtz and Lucas were at logger-heads over a lot of things. Some of them technical, some of them artistic. According to Kurtz they agreed they should go their separate ways before RETURN OF THE JEDI began, with Kurtz taking the opportunity to work with Jim Henson on THE DARK CRYSTAL.
However, since Kurtz was so intricately involved in the evolution of STAR WARS into something bigger, he's a great source. According to him, the outlines for the nine movies went like something this:
- EPISODE 1: Was to focus on the origins of the Jedi Knights and how they are initiated and trained
- EPISODE 2: Introduction and development of Obi-Wan Kenobi
- EPISODE 3: Introduction and life of Vader
- EPISODE 4: There were seven different drafts of the film. At one point, they pursued buying the rights to Hidden Fortress because of the strong similarities. At one point, Luke was a female, Han was Luke's brother, Luke's father was the one in prison (interesting point for some debates) and the film featured 40 wookies
- EPISODE 5: Once written, the screenplay of Empire is almost exactly what is seen on screen. The only cut scenes were those involving wampas in the rebel base (cut because of time and unsolved technical glitches) and about two minutes of Luke/Yoda Jedi training with no real dialog.
- EPISODE 6: Leia was to be elected "Queen of her people" leaving her isolated. Han was to die. Luke confronted Vader and went on with his life alone. Leia was not to be Luke's sister.
- EPISODE 7: Third trilogy was to focus on Luke's life as a Jedi, with very few details planned out.
- EPISODE 8: Luke's sister (not Leia) appears from another part of the galaxy.
- EPISODE 9: First appearance of the Emperor.
(I believe he went into greater detail in an issue of SFX magazine, but I've been unable to find a copy of the text on the internet... I bet it's there somewhere!)
There's also a great interview with Kurtz here, where he goes into a bit more detail:
The one story thread [in Return of the Jedi] that got totally tossed out the window, which was really pretty important I think, was the one of Vader trying to convince Luke to join him to overthrow the Emperor. That together they had enough power that they could do that, and it wasn’t him saying I want to take over the world and be the evil leader, it was that transition. It was Vader saying, “I’m looking again at what I’ve done and where my life has gone and who I’ve served and, very much in the Samurai tradition, and saying if I can join forces with my son, who is just as strong as I am, that maybe we can make some amends.” So there was all of that going on in Jedi as well, that was supposed to go on. So the story was quite a bit more poignant and the ending was the coronation of Leia as the queen of what was left of her people, to take over the royal symbol. That meant she was then isolated from all of the rest and Luke went off then by himself. It was basically a kind of bittersweet ending. She’s not his sister that dropped in to wrap up everything neatly. His sister was someone else way over on the other side of the galaxy and she wasn’t going to show up until the next episode.
Read more: http://www.filmthreat.com/interviews/8/#ixzz2Fcy0psgB