Perhaps the most fascinating "what if" of the Sequel Trilogy, Colin Trevorrow's version of the Skywalker Saga finale, titled Duel of the Fates, has proven to be a treasure trove of plot alternatives for those Star Wars fans left unsatisfied by The Rise of Skywalker. The plot, concept art and other critical details have leaked surrounding the first pre-production effort for Episode IX.
One of the more interesting differences between the scripts produced by J.J. Abrams and Trevorrow is the portrayal of Kylo Ren/Ben Solo. It's well established how his story ends in Abrams' version. Ben, like his grandfather, turns to the Light Side at the last moment in order to save his companion, Rey, and this sacrifice results in the Emperor's death. In this case, however, the death of the Emperor comes at the hands of the person who is saved (Rey), and not at the hands of his apprentice, as was the case with Darth Vader in Return of the Jedi.
It's notable that Abrams and Disney were more concerned with the symmetrical nature of the Star Wars universe when ending the Skywalker Saga, as there are parallels between the sequel trilogy and the original trilogy that are undeniable. This isn't a bad thing, but it's clear that Disney's priorities for Episode IX shifted greatly while the film was in development.
There is no doubt, however, that what audience were denied was also incredibly interesting. Rey with a double-sided lightsaber built from the remains of Anakin's saber and her staff, as well as bolstered roles for both General Hux and Leia, were among the story threads and set pieces cut out. Even a new design for Kylo was changed.
His portrayal in Duel of the Fates included a different mask than the one in The Rise of Skywalker, which was his destroyed mask repaired with red fillings. In Trevorrow's version of events, Kylo wears a mask which combines the styles of Boba Fett and Darth Vader, two of the most visually-unique villains in the Star Wars universe. In this version of the Skywalker Saga finale, Kylo is given much more time to fully explore the depths of the Sith lifestyle, and it reflects in this design choice. If nothing else, this change would have solidified Kylo's evolution to the Supreme Chancellor role after he murdered his former master, Snoke, in the saga's previous entry.
The helmet would also carry a much more significant metaphorical meaning in Duel of the Fates. Being modeled after Fett and Vader, Kylo would be simultaneously connecting his Sith legacy to the pasts of his father and mother. The Mandalorian aspect of the armor traces back to Fett, who was an enemy to his father, Han Solo, in the original trilogy. While the Mandalorian are more resolute and stoic, the armor itself has a different meaning to Star Wars fans. To them, it represents a life free from the law, and this fits right in with Fett and Kylo's lives.
On the Vader side of the equation, the connection here is much stronger to Kylo's Light Side leanings. Vader is the father of Luke and Leia, so both Kylo and his mother carry Vader's legacy. This is a subtle storytelling factor that is entirely absent from the Kylo's final design in The Rise of Skywalker. Through this interpretation, the Duel of the Fates armor could have served the purpose of being an indicator to the viewers that Kylo, like his grandfather before him, is torn by the conflict within him. If this was the only thing that was carried over to the official version of Episode IX, it would have made Kylo's character more interesting.