Cyborg's role in Justice League was far more important in Zack Snyder's original cut of the movie. Cyborg was supposed to have far more story and character development in Snyder's original version, where Snyder said Cyborg was the "heart" of the movie, but instead, Cyborg's role was drastically reduced and altered for Joss Whedon's theatrical version of Justice League,
Snyder's DC Extended Universe approach always drew intense debates over the way it decided to introduce new characters. Instead of giving each character their own solo film first, he was going to bring everyone in as a side character to a 5-part Superman story, with spin-offs telling their solo stories later.
Superman, as the main character, got his full origin story in Man of Steel. Batman is one of the most popular characters in the world and everyone knows his story, so Snyder introduced him in Batman v Superman: Dawn of Justice by showing us the origin story we know, followed by a prologue that shows how he fell, followed by a movie that charts his return to heroism, inspired by the franchise focal point, Superman.
Wonder Woman also got a Batman v Superman: Dawn of Justice cameo, followed by Wonder Woman, her smash hit origin story. Aquaman and Flash were supposed to get a similar treatment to Wonder Woman, but Cyborg, while he had a solo movie on the way at the time, was also going to take the spotlight in Justice League.
Cyborg's original role and significance may have been erased, but thanks to the slow drip of information about the Snyder Cut and Cyborg's role in it, we now know exactly what Cyborg's original Justice League story was, and how it made him the most important character.
Justice League Was a Cyborg Origin Movie
Justice League introduced Cyborg, Aquaman, and The Flash to the big screen for the first time (after all three cameoed in Batman v Superman: Dawn of Justice and Flash had another quick cameo in Suicide Squad). While Snyder and writer Chris Terrio gave all three characters more backstory and character development that was cut from Joss Whedon's theatrical release, Cyborg didn't merely lose some character development, he lost an entire origin story.
Thanks to behind the scenes footage, leaked scenes, information released by Zack Snyder, and more, we now know more about Cyborg's overall arc in the Snyder Cut, and he truly is the heart of the movie, as it revolves around his origin story. Snyder even developed a unique camera multi-camera rig explicitly so he could capture Cyborg's new origin scene in a special way. The scenes captured with that rig were obviously removed.
The movie would have shown him as a star football player, scenes with his mother, the car crash that killed her and severely injured him, a new look at Cyborg's creation at Star labs (after getting a tease in his Batman v Superman: Dawn of Justice cameo), Cyborg discovering his powers, seeking acceptance from the world, embracing his powers and role as a hero, and more. Nearly every element from a typical superhero origin story could be found in his Justice League arc.
It's no wonder Cyborg drew criticisms for being a shallow character and Justice League lacked any kind of dynamic plot, the character arc the whole thing was built around was entirely absent from the final product.
Cyborg Was Isolated From Humanity
A major theme for Cyborg would have been his isolation from humanity. After being a star football player, Victor Stone would have experienced a horrific accident, losing his mother and most of his body. Fisher also says Snyder and Terrio intentionally gave Cyborg a story intended to give representation to people with disabilities.
Naturally, he wasn't comfortable going out in public unless it was under the cover of night while wearing a hoodie, but even then he was overwhelmed with new knowledge and abilities he had gained with the mother box. The knowledge in his head alone must have been overwhelming, suddenly becoming connected to every network and every database in the world, and possibly beyond.
Making matters worse, he comes from the same thing that powers the army of Apocolypse. He looks very much like a parademon, and may have even had to battle programming or messages from Darkseid or Steppenwolf. Not only would this make him uncomfortable in his own body, but could also be a barrier to him coming together with the Justice League, who would develop obvious trust issues over someone bearing such a relationship to the villain.
The Snyder Cut also had Silas Stone dying, severing Victor from his last remaining link to mankind (tenuous as that relationship may have been). It's only through the theme of unity and finding a new home with the Justice League that he's able to stay anchored to Earth instead of being drawn to the will of the Motherboxes.
Cyborg Was Tempted by the Mother Box and Darkseid
The pull for Cyborg to abandon Earth and the Justice League was strong in the Snyder Cut. The Mother Box, and possibly Darkseid himself, were calling to him. He would have been subjected to two known visions or dream sequences. First, on the Kryptonian ship he would have had Justice League's Knightmare vision, seeing Darkseid on a throne, devastating the Earth with the Mother Boxes.
The second would have been while he was trying to separate the Unity of the Mother Boxes during the final battle. Snyder revealed an image of a Mother Box vision where Victor would have seen an image of himself standing with his mother and father, his body fully flesh again. Through the Avatar of his parents, the Mother Box would have called to him: "My poor boy I can make you whole again, Victor," tempting him to abandon his mission and his friends.
Cyborg Was The Only Hero Who Could Defeat Steppenwolf
Not only did Cyborg have a full, complex character arc, but he's also the most central character to the plot. His knowledge is what makes it possible for them to resurrect Superman, and, as Zack Synder recently revealed, "The Unity can only be prevented from the inside," and Cyborg is also the only one who could interface with the Mother Boxes to separate them, making him indispensable to the plot.
So Cyborg's arc is about him finding his humanity again and finding belonging with the League, and that connection is what allows him to save the world, not just by being the savior of its most important hero, but also by being the only person with the knowledge to resurrect Earth's most powerful hero or the power to defeat Steppenwolf and Darkseid's plan at the end.
Silas Stone Originally Had the Closing Monologue
To cap it all off, Silas Stone was supposed to provide the ending monologue via a recording he had left behind for his son before his death. The lines instead were given to Lois Lane. Given the change in character and the fact that she was writing instead of recording a video, it's likely the lines were modified, but the general theme about finding light in darkness is something Silas certainly would have talked to his son about, as well as the importance of embracing his abilities as a hero.
Among Justice League's many criticisms, Cyborg's relatively shallow character and the movie's overall lack of a driving thematic plot were among them. Looking at all the cut Cyborg story it"s clear he's the missing puzzle piece. His origin story and overall arc were at the heart of the movie, and by neutering his story, Justice League was also neutered.
There's a long list of poor choices when it comes to the way Justice League was modified, ranging from Henry Cavill's CGI upper lip to the complete removal of Darkseid, but the dismantling of Cyborg's character, and way that change sterilized the rest of Justice League's plot, is possibly the most significant and damaging of all the changes.