Future Star Wars movies could still feature Luke, Leia, and even Anakin Skywalker. Star Wars: The Rise of Skywalker is seen as the end of the Skywalker saga, the trilogy of trilogies that began with George Lucas and ended with J.J. Abrams. It focuses on the fall of Anakin Skywalker, his redemption, and his ultimate legacy.
In theory, Star Wars: The Rise of Skywalker should be the end for the heroes of the original trilogy. Only Lando Calrissian is left; Han Solo was murdered in Star Wars: The Force Awakens, Luke Skywalker sacrificed himself in Star Wars: The Last Jedi, and Leia Organa slipped away in the last film. For all that's the case, though, the final scenes of Star Wars: The Rise of Skywalker confirmed both Luke and Leia are now Force Ghosts - meaning they could well return.
Back in 1991, writer Timothy Zahn effectively launched the old Star Wars Expanded Universe with his fondly-remembered "Thrawn Trilogy". Zahn wanted to demonstrate Luke Skywalker's strength and ability, and as such he decided to strip away some of his support network, forcing the young Jedi to stand on his own two feet. He did this by revealing Force Ghosts can only stick around for a short period of time; Luke was left heartbroken and alone when Obi-Wan Kenobi's Force Ghost appeared to him one last time to say goodbye. "There is no magic wand to make everything better," Luke realized, "even for a Jedi."
But the Thrawn Trilogy isn't canon anymore. Back in 2012, Disney purchased Lucasfilm for $4.05 billion; they swiftly decided to render the old Expanded Universe non-canon, or "Legends," simply because it gave them more narrative room for maneuver. As a result, it's become clear Force Ghosts in the Disney-era work a little differently; there are no longer any limits to how long they can manifest. Master Yoda appeared to Luke in Star Wars: The Last Jedi, and the novelization of Star Wars: The Rise of Skywalker reveals both Obi-Wan Kenobi and Master Yoda had taught Leia the ways of the Force during the long years of her brother's exile.
This, naturally, raises the possibility that the Skywalker saga is far from over. Luke and Leia may be dead, but they can return as Force Ghosts at any time, perhaps as mentors to the next generation of Jedi. They are truly immortal, and as such they can continue to influence the galaxy even decades after they are gone. What's more, Star Wars: The Last Jedi featured one scene in which Master Yoda demonstrated he could still influence the corporeal world as a Force Ghost; the same is, consequently, true of Luke, Leia, and even Anakin Skywalker himself.
Of course, Lucasfilm may be a little more reluctant to feature the Force Ghost of Leia Organa in any future Star Wars movie. That would probably require CGI, the same kind of approach that brought a young Leia back for Rogue One: A Star Wars Story, but fans are vocally opposed to such an approach. What's more, it's important to remember than many of the decision-makers at Lucasfilm considered the late Carrie Fisher to be a good friend, and they truly would want to honor her memory. Thus it's unlikely Leia will return in any hypothetical Star Wars 10, or indeed in any story set after Star Wars: The Rise of Skywalker; but the same cannot be said for Luke and his father, Anakin.