Johnson was in part following a plan laid out by George Lucas himself.
Indeed, Lucas had a hand in the story of The Last Jedi...
Johnson's movie used many of the concepts Lucas first had in mind for Episode VII, specifically the story of an older, broken Luke training a young, Force-sensitive disciple named Kira (later renamed "Rey").
We know that when Disney bought Lucasfilm and the rights to Star Wars, it also purchased Lucas' ideas for the Sequel Trilogy, which he had been planning to make himself before he decided to sell the franchise. He wrote a story treatment and planned to participate behind the scenes as a consultant when it came time to craft the story for what would eventually become The Force Awakens.
The earliest concepts for the Jedi temple on the planet that would become Ahch-To were ironed out during these meetings as well as visualizations of what an older Luke and young Kira would look like. Lucas even approved the design for a bell-shaped structure for the Jedi temple.
Among the early designs tossed around for Luke were ones that looked pretty close to what we got in The Last Jedi as well as one or two that were a bit radical, such as a bald Skywalker who looked closer to a Buddhist monk than a shaggy hermit. Kira's concept didn't stray too far from the plucky young heroine we got in Rey.
Lucas' biggest contribution to The Last Jedi was the Luke/Rey story itself, which he'd originally planned for Episode VII, as confirmed by Lucasfilm Story Group member Pablo Hidalgo on Twitter (via Comicbook.com). The idea played out much like what we saw in Johnson's movie: Luke is down and out on a distant planet thirty years after the fall of the Empire. A new hope named Kira finds the old Jedi Master and they begin her training. We would have seen Luke struggling with his failure to stop the Jedi Killer (renamed "Kylo Ren") from destroying his Jedi Academy.
"[Luke] always had this potential dark side within him, being that his father was Darth Vader," explained Lucasfilm executive creative director Doug Chiang of the character's arc in the early days of Episode VII. "So he is really struggling with that. He ended up secluding himself in this Jedi temple on a new planet, and he's just there meditating, reassessing his whole life. Gradually, over the arc of the movie, he rediscovers his vitality and comes back to himself."
Eventually, Luke would have regained his mojo while teaching Kira the ways of the Force and rejoined the fight. That's basically the skeleton of The Last Jedi's plot.
Lucas also had a part in crafting the Kylo Ren storyline, including the fact that the villain was to be revealed as Han and Leia's son.
Hidalgo also mentioned a character named "Skyler" in his tweets who eventually became Finn.
It was screenwriter Michael Arndt's idea to push this story to Episode VIII. While Arndt was eventually replaced by Abrams and Lawrence Kasdan, who co-wrote the final draft of The Force Awakens, he came up with the basic structure of the plot, which involved the search for Luke Skywalker and "a victory lap" for Han Solo.
We'll never truly know what Lucas intended to do with the sequels, yet he was a part of the process early on and the Sequel Trilogy still carries part of his vision.
At least one person involved with the sequels, Mark Hamill himself, is a little disappointed that Lucas hasn't been more involved with the franchise going forward, according to an interview with Metro.
“What I wish is that [Disney] had been more accepting of [Lucas'] guidance and advice," Hamill told Metro. "Because he had an outline for ‘7,’ ‘8,’ and ‘9’. And it is vastly different to what they have done.”
