The original Star Wars trilogy could have turned out very differently if the original Star Wars film didn't succeed the way George Lucas had hoped. Even if the film didn't do very well, he still wanted to make a sequel, so on top of the idea he already had envisioned, there was also a plan B for a smaller scale movie. Just in case the box office returns of the first movie were low and he couldn't get the budget he needed to bring his own idea for Empire Strikes Back to life, he had a second option to fall back on.
According to Screen Crush, Lucas hired science fiction novelist Alan Dean Foster to write up the the backup plan for a smaller scale follow up in the form of a book. It was titled Splinter Of The Mind’s Eye. In an interview with the author he said:
“I was originally brought in while ‘Star Wars’ was still in production, on a two-book contract to do the novelization of the film, but he also instructed me to write a novel that could be filmed on a low budget."
The story for the film focused more on Luke and Leia, mostly because Mark Hamill and Carrie Fisher were the only two stars contractually on board to reprise their roles at the time. It featured the two characters on their way to an important meeting with representatives of a system that might join the Rebellion. On the way there they crash land on a swamp planet called Mimban. While stranded there they discovered that Mimban is a strategic mining outpost for The Empire. Apparently The Empire was trying to find something called a Kaiburr crystal. It ended with a climactic lightsaber battle between Darth Vader and Princess Leia with Luke pinned down by a rock. It's then explained that Luke eventually gets into the fight, severing Vader’s right arm before sending him down a "seemingly bottomless pit." Wow! How crazy would that have been?! Han Solo wasn't included in the story, and was only mentioned twice. Foster explained:
“I couldn’t use the characters of Han Solo and Chewbacca. Chewbacca because he’s Han Solo’s buddy and Han Solo because Harrison Ford hadn’t yet signed on to do anything related to future developments of ‘Star Wars.’ It was kind of liberating. The story idea I had in mind focused on Luke, Leia and, in the background of course, Darth Vader. And working a third major character into the storyline would have been a little awkward. It was less that I had to worry about.”
The Skywalker family and relationships weren't all figured out at this point, and the story doesn't really fit the established universe canon. Like the relationship between Luke and Leia, where at one point in the book they have to get up close to each other for warmth. The scene is described as follows in the book:
“Awkwardly pressed up against him, the Princess seemed to take no notice of their proximity. In the dampness, though, her body heat was near palpable to Luke and he had to force himself to keep his attention on what he was doing.”
Yeah, so that's a little awkward knowing what we know now. The author had no idea they were siblings when writing the book though. It wasn't even until Lucas wrote the second draft of the script for Empire that he had the idea to make Darth Vader Luke's dad. Foster went on to say,
“What I was working from, basically, was the kiss in ‘Star Wars,’ before they swing across the gap. That didn’t strike me as a particularly brotherly-sisterly kiss at the time.”
As you know Star Wars succeeded, and Lucas used all of the money that he made from it to give us Empire Strikes Back. If Empire would have failed, Lucasfilm would have died with it.
It would have been really interesting to see how the Star Wars universe would have evolved if this is the movie that would have been made. It would have definitely changed everything that we know and love about the franchise. As interesting as this story is, I'm so happy that Lucas committed himself to make Empire.