Just like in Batman v Superman: Dawn of Justice, Zack Snyder did a lot of universe building for Justice League. From putting its six starring heroes on new paths following their battle against Steppenwolf to introducing characters like Mera and Deathstroke, there were a lot of ways that Justice League directly adapted DC Comics mythology, and a few indirect ways as well. Falling into the latter category, it turns out that Snyder planned on featuring Ryan Choi, a.k.a. The Atom, in the movie.
Earlier this year, eagle-eyed fans noticed when re-watching Justice League that Ryan Choi's name was displayed on the side of a computer screen, revealing that in the DC Extended Universe that Ryan is an employee at S.T.A.R. Labs. Fast-forward to now, a fan found a Justice League promotional picture featuring Silas next to an unnamed Asian scientist, and Zack Snyder confirmed on Vero that this was Ryan, played by Star Wars: The Last Jedi's Orion Lee. However, this scene was ultimately one of many cut during the post-production/reshoots phase once Joss Whedon came aboard to finish Justice League. Whether or not Ryan would have actually been referred to by name is unclear, but Snyder definitely intended for him to exist in this continuity.
Introduced in 2006, Ryan Choi is the fourth individual to use the Atom identity, following behind Al Pratt (the Golden Age Atom), Ray Palmer (the most well-known of the bunch) and Adam Cray. Originally Ray's protege, Ryan came to the United States to take over Ray's teaching position when Ray disappeared, and after following a series of clues, Ryan found the size and density-manipulating belt Ray used to fight crime as The Atom. Ryan subsequently succeeded Ray as the new Atom, and although he was killed years later by Deathstroke, he later returned in the DC Rebirth continuity, becoming The Atom once more. Outside of the comics, Ryan's most prominently appeared in Batman: The Brave and the Bold.
It sounds like Ryan Choi's appearance in Justice League would have been a minor appearance at best, which is fine given everything that movie had to juggle. Still, including him as a S.T.A.R. Labs employee was a wise choice, not just because he retained his scientific background, but that could have laid the groundwork for the shrinking technology he uses in the comics to be included in Justice League 2 or another DCEU entry. Alas, it was not to be, but maybe there will come another day when Ryan can be featured in a different DC movie.
The DCEU will continue on December 21 with the release of Aquaman, and Shazam! and Wonder Woman 1984 will follow on April 5 and November 1 of 2019, respectively. You can also look through our DC movies guide to find out what other projects are in development for the superhero franchise.