Star Wars: The Last Jedi emphasized the failings of the Jedi, with Luke trying to bring an end to the order. Yet, by the end of the sequel trilogy, the Jedi are once again redeemed through Rey and seen as the true balance of the galaxy. While the moment when famed Jedi from the past supporting Rey as she fought Palpatine is satisfying, the sequels missed the opportunity to introduce Rey to the Gray Jedi, a concept from Legends.
After Disney acquired Star Wars in 2012, the studio wiped the slate clean with the somewhat-messy timeline, making it so only central works -- namely, the movies and Star Wars: The Clone Wars -- were canon. While that cleared up the confusion with conflicting novels, comics and video games, it, unfortunately, removed a lot of interesting concepts from Star Wars. One of them is the Gray Jedi.
In the Old Republic era of old canon, the Gray Jedi were a faction of Force-users who sat in between the light and dark. Often being ex-Jedi students, these people respected the use of the Force while not being bound to the strict rules of the Jedi Order or Sith. They could use both light and dark force abilities, but without fully committing to a single side. Because of this, they were looked down upon by many, as each side of the Force-users view there being either good or evil with no gray areas.
While the Sith are undoubtedly the evil side of the Force, Luke raises some good points about the failings of the Jedi in The Last Jedi. He says to Rey that "At the height of their powers, they allowed Darth Sidious to rise, create the Empire, and wipe them out," suggesting the Jedi teachings are heavily flawed. And supporting this further is Luke's training in Star Wars: Return of the Jedi, as Luke refuses to kill his father because of his attachment to him. As Obi-Wan believes attachment is forbidden and will lead to the dark side, he says, "Then the Emperor has already won." However, in the end, Luke's attachment to his father is what saves him and the galaxy.
And so, this paired with the idea of creating balance in the force means The Last Jedi had the perfect opportunity to reintroduce the Gray Jedi, as it doesn't get more balanced than being in the middle. At the same time, it's clear that Disney and even George Lucas have always believed that balance comes from eradicating the Sith.
But either way, there's still the chance of Gray Jedi being introduced into the new canon. The High Republic era of Star Wars is slowly being fleshed-out with upcoming shows like Star Wars: The Acolyte, which could include both new and old lore.