After the prequels had come out, Star Wars had easily become the biggest media franchise in the world. As George Lucas continued to expand his franchise in the digital age, he was also preparing for retirement with plans to give the keys to Lucasfilm to someone new. The result was of course the Disney buyout and Kathleen Kennedy was made the new President of Lucasfilm (via Deadline).
At this time, Lucas' plans for the sequel trilogy were a go but he wanted a new generation of filmmakers to make them in his place. Unfortunately, things didn't go as planned and Disney chose not to use Lucas' writing material, a decision both Lucas and Disney CEO (at the time) Bob Iger would later express disappointment in. The many concepts from Lucas' sequel trilogy were really exciting and it's sad they'll never see the light of day but luckily the recent book Star Wars Archives: 1999-2005 offers a lot of insight into them.
Smaller Time Jump
A notable decision in the sequel trilogy was to make it take place a whole 34 years after Return of the Jedi. However, George Lucas had different plans, of course. Lucas' Episode VII would have picked up 20 years following Episode VI. The entire scope of the galaxy is completely different following the Rebels' victory in VI.
The time jump would have also given the film the opportunity to explore a period where there's no major government power (so the galaxy is kind of like the wild west). This way of looking at the world would later be put to use in The Mandalorian, with that show really emphasizing the power vacuum left after the end of the Galactic Civil War.
Luke's Jedi Academy
As many fans feel, Episode VIII kind of squandered Luke as a character, subverting people's expectations and turning him into a flawed character. Lucas had a very different direction that probably would have gone well with audiences.
Per Star Wars Archives: 1999-2005, Lucas mentioned "[...]Luke is trying to restart the Jedi. He puts the word out, so out of 100,000 Jedi, maybe 50 or 100 are left. The Jedi have to grow again from scratch, so Luke has to find two-and-three-year-olds, and train them. It’ll be 20 years before you have a new generation of Jedi.” This version would have had Luke portrayed as his heroic self instead of a wanderer who was lost in self-exile.
Leia Is The Chosen One
The question of who is The Chosen One has been pondered by many, both in-universe and in real life. The debate has been between Anakin, Luke and Leia forever but George Lucas finally squashed the age-old question. It's Leia. Carrie Fisher was intended to return and headline this trilogy. Lucas made it important to note “I had planned for the first trilogy to be about the father, the second trilogy to be about the son, and the third trilogy to be about the daughter and the grandchildren."
This trilogy would have followed her as she navigates herself through the new political atmosphere in the galaxy, following Palpatine's demise. Like in the Legends continuity, Leia was trying to rebuild a democratic government through the New Republic. Fans would definitely have loved this, giving Fisher an even bigger role then she had in Disney's trilogy.
Children of the Force
Vader's grandchildren were always meant to be the focus of the sequel trilogy, according to George Lucas. Luke wasn't meant to have kids but this trilogy would have followed Han and Leia's offspring.
However, it is unknown if they would have been original characters or if Lucas would have just used the Legends' history with Jacen, Jaina and Anakin Solo. The two new main characters of this trilogy were slated to be Kira and Sam. Their relation to the Skywalker bloodline is unknown though.
Death Star Exploration
One major plot point that stayed for the Disney-made sequel trilogy was the wreckage of the Death Star on an aquatic planet (as seen in it makes no sense in Episode IX, for the Death Star to still be somewhat together since the whole thing was obliterated. However, there were also plans for this to appear in George Lucas's Episode VII. Drawn by Lucasfilm veteran Doug Chiang, these pieces show the Millennium Falcon investigating the wreckage of the second Death Star underwater.
Other keyframes include a female diver (possibly Kira, the prototype for Rey) exploring the Emperor's tower from Episode VI and another one of the same character treading next to a turbo-laser turret, covered in algae. It's unknown how this would have factored into the plot but it would have been super cool to explore familiar abandoned locations from the Original Trilogy.
Midi-Chlorians
A plot element that many balked at in 1999, was the inclusion of midi-chlorians in The Phantom Menace, a unit to measure a person's ability in the force. Had George's sequel trilogy happened, the films would have explored the midi-chlorians more and the biological science behind the force.
In James Cameron's The Story of Science Fiction series, Lucas said “[The next three Star Wars films] were going to get into a microbiotic world. But there’s this world of creatures that operate differently than we do. I call them the Whills. And the Whills are the ones who actually control the universe. They feed off the Force."
Darth Maul Is The Big Bad
Before finally dying in Rebels in 2017, Darth Maul could have lived on to be in the sequels, appearing as this trilogy's main villain had things gone differently. From his first appearance in The Phantom Menace, Darth Maul became a fan-favorite character. With his iconic double-bladed saber, he would have made a worthy adversary to Luke Skywalker.
As a natural progression of his character, Maul would have taken over the entire galaxy's underworld. He would have been working alongside the Imperial Remnant as they attempt to crush the Republic. He also would have had Darth Talon as his apprentice, a concept that seems to be taken right out of the canceled Battle of the Sith Lords game.
Luke's Fate
Luke's fate in Lucas' Episode VII would have been very different with his character never actually dying. Only that he would die at the end of Episode IX instead. In an interview with Mashable, Mark Hamill said "I happen to know that George didn't kill Luke until the end of IX after he trained Leia."
Luke's role in this trilogy would have sincerely delighted fans and it's a shame Hamill couldn't portray the character the way he was intended to be. Thankfully, Disney's grand The Book of Boba Fett gave him that opportunity.
A Different Direction
In a way that The Force Awakens heavily cashed in on nostalgia, George Lucas' plan was to do something very different to differentiate the feel of each trilogy. In an interview with Charlie Rose, Lucas stated "I worked very hard to make them completely different, with different planets, with different spaceships – you know, to make it new”.
In his interview with James Cameron, Lucas also stated "If I’d held onto the company I could have done it, and then it would have been done. Of course, a lot of fans would have hated it, just like they did Phantom Menace and everything, but at least the whole story from beginning to end would be told." It's disappointing that fans will never see Lucas' Star Wars vision fully realized but at least there is solace knowing his protégé, Dave Filoni, is continuing his legacy at Lucasfilm.
