Disney's Star Wars sequel trilogy would have been very different if one major fan theory had been true: that Supreme Leader Snoke was Darth Plagueis. One of the new characters introduced, alongside Rey, Finn, Poe, and Kylo Ren was Supreme Leader Snoke, played by Andy Serkis. Snoke was established in The Force Awakens, despite only having two scenes via hologram, as the Supreme Leader of the villainous First Order and the master of the dark apprentice Kylo Ren. Snoke then went on to have a larger role in the second sequel film, Star Wars: The Last Jedi, which subverted many a theory directed towards the character after his brief appearance in The Force Awakens.
The main theory surrounding Snoke's character after the release of Star Wars: The Force Awakens was that he was Darth Plagueis, the master of Darth Sidious mentioned in Star Wars: Episode III - Revenge of the Sith. Despite a substantial portion of the Star Wars audience placing a lot of stock in this theory, it was disproved by both The Last Jedi and the final film in the sequel trilogy, Star Wars: The Rise of Skywalker. In the infinite multiverse, there is surely a reality in which these theories were confirmed and Snoke was indeed Darth Plagueis, which would have spelled a very different story for the Star Wars sequel trilogy, and likely changed most of its major moments.
Why Star Wars Fans Thought Snoke Was Darth Plagueis
The main basis behind the theory that Snoke was Darth Plagueis came from Plagueis' first mention in Revenge of the Sith. In a scene in which Emperor Palpatine is attempting to lure Anakin Skywalker to the dark side, Plagueis is mentioned to have had unnatural abilities granted to him by the dark side of the Force, with one being that he could influence midichlorians to create life. It is mentioned by Sidious, Plagueis' apprentice, that the Sith Lord craved immortality above all else and attempted to use this power in order to cheat death. Upon the release of Star Wars: The Force Awakens, many believed that Snoke was indeed Plagueis and that the old Sith Lord had managed to achieve his lifelong goal despite his supposed death at the hands of Palpatine. This, combined with the strong affinity with the dark side of the Force that Snoke was said to possess, led many to believe he was Darth Plagueis in one way or another.
Snoke (Probably) Wouldn't Die In Last Jedi If He Was Plagueis
If this theory had been confirmed, the biggest and most obvious change to the Star Wars sequels would have been that Snoke likely was not killed in Star Wars: The Last Jedi. One of the biggest expectation subversions of Rian Johnson's film was the sudden death of Snoke at the hands of Kylo Ren, putting to bed any rumors that Snoke was an important character to the trilogy as a whole. This disappointed a lot of viewers, especially those who let their expectations run unchecked, like those who suspected Snoke was Plagueis. That being said, if this theory was true, it is highly likely that Snoke would have survived into the final installment. Given the mystery of Plagueis and the fact that he is one of the few characters confirmed to be as powerful (if not more so) than Palpatine, it would have been fitting for him to be the main villain of the sequel trilogy, therefore surviving through the events of The Last Jedi, unlike Snoke.
How Kylo Ren's Story Would Change If Snoke Was Plagueis
One of the other major changes to the sequel trilogy if Plagueis had lived on through Snoke would have been Kylo Ren's journey. As mentioned, Kylo kills Snoke in Star Wars: The Last Jedi, taking control of the First Order and overcoming his abusive master. If Snoke was Plagueis, though, it is likely that this situation would have been very different, and would have also created a different future for Kylo himself. Firstly, if Plagueis had been confirmed to be Snoke, it would mean the sequel trilogy was much more conventional, without the subversion of expectations that The Last Jedi employed. Because of this, it likely would not have led to Snoke's death in the second installment, thus stifling the beginning of Kylo's redemption arc.
Kylo's murder of Snoke is what truly begins his eventual redemption in The Rise of Skywalker as his relationship with Rey is the catalyst for his actions. This would have then altered Kylo's journey in The Rise of Skywalker, resulting in him potentially dying as a villainous Sith as opposed to being put on the path back to the light side of the Force by murdering Snoke. Kylo would have likely remained the main antagonist for Rey to overcome, but would have still been second in command to Darth Plagueis, who would have been the overall villain of the trilogy, emulating the original trilogy with Vader and Palpatine.
Snoke Being Plagueis Would Likely Mean Rey Wasn't A Palpatine
One of the main storylines of the sequel trilogy surrounded Rey's bloodline, and eventually resulted in the revelation that Rey was a Palpatine before she takes on the adoptive name Skywalker. If Snoke was revealed to be Darth Plagueis, it would have been very unlikely for Rey's Palpatine heritage to come to fruition. Due to Snoke's untimely death in The Last Jedi, Palpatine was controversially brought back as the sequel trilogy's main villain. In order for Palpatine's return in The Rise of Skywalker to make more sense, the connection between Rey and Palpatine was conceived. However, if Plagueis was indeed the main villain of the sequel trilogy, he would have likely survived until the final film, meaning the need for Palpatine's resurrection was removed. This would subsequently remove Rey's heritage and connection to the Sith Lord, resulting in a very different Episode IX for the sequel's main character.
Would Palpatine Return If Snoke Was Darth Plagueis?
As per the previous point, it would be unlikely for Palpatine's return to have happened if Snoke was Darth Plagueis. As Revenge of the Sith established Plagueis as Palpatine's master, having him as the main villain of the sequel trilogy would have been a logical step-up in terms of villainous threat for the Star Wars universe. While the argument over who is more powerful between Plagueis and Sidious has not been formally answered, having the villain of the sequel trilogy being the long-lost master of the villain of the first two trilogies would have been a compelling concept. Because of this, rehashing Palpatine would not have been necessary and would not have retroactively hurt Anakin Skywalker's character arc like The Rise of Skywalker did, instead focusing on a new villain for the new heroes to overcome.
How The Star Wars Sequels' Ending Would Be Different
The main way in which the Star Wars sequels' ending would have been affected by Plagueis' inclusion is in its finality. One concept that could have been introduced should Plagueis have been the sequels' main villain is a bleaker "ending" not unlike the prequel trilogy. Despite the fact that the prequel trilogy was conceived after the originals, the two work together very well as a six-episode story. If a threat as important to the overall status quo of the Star Wars universe, due to his Sith training of Palpatine, had been introduced as the main villain of the sequel trilogy, the three films could have been created in a way that ends with Plagueis winning, setting up another trilogy in the process. This would have drastically changed the sequel trilogy's ending, which culminated fairly neatly with The Rise of Skywalker despite the illogical plot. Not only would this potential ending have been more compelling and given both Plagueis and the new characters more time to grow in another three movies, but it would have continued one of the core George Lucas Star Wars ideologies due to its parallels with the rest of the franchise: "It's like poetry, sort of. They rhyme. Every stanza kind of rhymes with the last one."
Why It's Bad That Snoke Wasn't Darth Plagueis
There are many arguments on both sides as to why it was a good and bad thing that Snoke was his own being and not Darth Plagueis. However, Snoke being Plagueis would have allowed for a focus on a villain strong in the dark side of the Force that was potentially even greater than Sidious from the first two trilogies, which does make the fact that he wasn't ultimately a shame. Plagueis being the man who taught Sidious everything he knows and cheating death to rule the galaxy is an intriguing concept and would have likely resulted in a much deeper exploration of the dark side of the Force than Snoke allowed in the sequels as they were. Also, one of the most criticized aspects of the sequel trilogy was the needless return of Emperor Palpatine, which could have been avoided if Plagueis was the main villain of the trilogy. Furthermore, it is likely Darth Plagueis' return would have led to another trilogy that would have further fleshed out the sequel-era characters. Regardless, there are many arguments as to why it would have been good and bad for Snoke to have been Plagueis - a debate that will remain ongoing, given that the actual Star Wars sequel trilogy leaves this an open topic due to the way it approached the character.