WARNING: The following contains spoilers for Star Wars: The Rise of Skywalker, in theaters now.
Ben Solo died three times in Star Wars: The Rise of Skywalker: once at the hands of Rey, who healed his wound moments later; once when Palpatine threw him into a pit, and he climbed out of it by himself; and then when he gave up his Life Force to resurrect Rey. Although that last death stuck, Lucaswfilm could have left an emergency exit of sorts, hidden in the lore of Star Wars cartoons and the Visual Dictionary, that would allow them to bring Ben Solo back to life.
But why? Despite his villainous debut in The Force Awakens, Kylo Ren has proved one of the more popular characters, with a portion of fandom heering him on to redemption. In the lead-up to The Rise of Skywalker, the studio heavily promoted Ben Solo's story across all media, raising hope that, by the end of the Skywalker Saga, he would finally abandon the Kylo Ren persona, return to his family, atone for what he had done, and live happily with Rey.
The choppy editing of his death scene is so baffling that, from a certain point of view, it looks as if Ben's death was a last-minute decision executed in post-production. Furthermore, Kylo Ren was coded as a deeply traumatized, brainwashed and abused child of war, even more so than Finn or Hux, so his death has bleak, real-world connotations about reintegration.
But the filmmakers left a planet-sized loophole that could be used to bring Ben back from the dead: the planet Exegol, where he dies, is actually not only a scary place favored by the Sith, like Korriban or Mustafar, but part of the World Between Worlds nexus, a plane that connects every point of space and time through the Force, and that has featured prominently on the animated Star Wars Rebels and The Clone Wars.
The first appearance of the World Between Worlds was in The Clone Wars' "Mortis" arc, in which Anakin, Obi-wan and Ahsoka find a dark cube floating in space. The cube is actually Mortis, a planet where days cycle through seasons, inhabited by the embodiments of the Force: the Father (the Balance), the Daughter (the Light) and the Son (the Dark). Ahsoka actually dies in the storyline, but Anakin brings her back to life by channeling the Daughter's Force through his body -- an act that doesn't kill him, obviously. At the end of the third episode, the trio leaves Mortis... only to discover the planet has never existed, and that no time has passed since they disappeared.
The World Between Worlds is also a fixture of Rebels from the moment when Ezra Bridger acquires his first kyber crystal. However, to him, it manifests as a labyrinthine corridor through space, with portals that lead to different points in time and space. On Rebels, we discover the Emperor coveted access to the World between Worlds, because that would basically allow him to live forever and funnel the entire Force of the galaxy, and maybe even the universe. Ezra also uses one of the portals to pull out Ahsoka right before Darth Vader strikes her down, marking her second encounter with this plane. Yoda also travels in his Lost Missions to the Source planet, the place where the Living Force originated, and a celestial body that seemed to exist only for him.
We have theorized before about the mechanics of Palpatine's return by piercing the veil of the World Between Worlds with the help of kyber crystals. The short version is that the violent manipulation of kyber crystals, which are naturally atuned with the Force, can break time and space, opening a wound that would allow Palpatine to access this plane -- and that was the reason behind the repetitive construction of Death Stars and Death-Star technology. Palpatine died falling into the core of Death Star II, which contained a kyber crystal, so there's a chance he could have survived by clinging to the World Between Worlds before reemerging somewhere else -- in this case, Exegol.
However, Star Wars: The Rise of Skywalker: The Visual Dictionary includes an image of the Sacred Jedi Texts that Rey stole from the Ahch-To tree library. The pages show a "visualization of the Chain Worlds Theorem, also known as the Worlds Between Worlds," and a close-up points at Exegol as one of the planets that belong to the Chain Worlds. In other words, the perfect place for Palpatine's comeback through the World Between Worlds.
So, if Palpatine made it through the World Between Worlds (even though he was in sore need of a mani by the time of The Rise of Skywalker) there's no reason for Ben not to come back as well. That his Force ghost didn't appear to Rey at the end of Episode IX, unlike Luke and Leia's, could also hint that he's not actually dead.
Directed and co-written by J.J. Abrams, Star Wars: The Rise of Skywalker stars Daisy Ridley, Adam Driver, John Boyega, Oscar Isaac, Lupita Nyong'o, Domhnall Gleeson, Kelly Marie Tran, Joonas Suotamo, Billie Lourd, Keri Russell, Matt Smith, Anthony Daniels, Mark Hamill, Billy Dee Williams and Carrie Fisher, with Naomi Ackie and Richard E. Grant. The film is now in theaters.