Zack Snyder's Justice League originally included another hero in Ryan Choi, AKA Atom. The movie may be nearing the one-year mark since its release, but we're still regularly learning new things about some of the drastic changes that occurred to the "Snyder Cut" after the Joss Whedon led reshoots, and Ryan Choi is only another addition to the long list of removed characters and scenes.
We already knew the reshoots drastically altered the story and tone, including some character arcs being cut (Flash, Cyborg, and Aquaman), and other characters being removed outright (Iris West, Vulko, and Darkseid), so it's not a huge surprise to find out there was originally more there, although it is hard to imagine how it all came together. Some of the reshoots, such as the near-complete replacement of every Superman scene, shows just how pervasive of an effect the reshoots had, but it doesn't seem like any major characters were removed, and in the case of Ryan Choi, he definitely wasn't supposed to have more than a supporting part to play.
While it's understandable that fans would get excited about the prospect of another Justice League member getting introduced - especially one of Asian descent, the part likely would have been comparable to the size of other cut roles, meaning he would have been a minor supporting character at best, and definitely wouldn't have suited up to join the rest of the League.
As it stands, the only hint we have about his character is that he's a S.T.A.R. Labs scientist that works closely with Silas Stone, with the Ryan Choi confirmation only coming thanks to a fan asking Zack Snyder who he was after discovering him in an old promotional image - even IMDB simply credits him as "Star Labs Scientist." That means his part could simply have been on the same level as Carrie Farris in Man of Steel and Batman v Superman: Dawn of Justice; however, there's a number of images from S.T.A.R. Labs scenes that suggest at least one important scene there was cut, and since Cyborg was originally more important to the core of the story, and the Mother Boxes were important in both iterations, his part could have been more in the vein of Jena Malone's character in the Batman v Superman Ultimate Edition, which was cut from the theatrical release.
It's also been learned that Silas Stone originally died in the Snyder Cut, so Cyborg and the League would need another S.T.A.R. Labs liason, which is a role that would make a lot of sense for Ryan Choi, possibly even setting him up as a supporting character for the Flash or Cyborg solo movies.
More than anything, it seems like the character's name and role with S.T.A.R. Labs were mostly an Easter egg cameo to establish a deeper connection to DC mythos. It definitely would have been brief, but that doesn't necessarily mean it was inconsequential. Giving cameos to future Justice League members is usually a little more than a wink to fans, but sans any rumors of an in-development Atom project from Warner Bros., that's likely all it will be for the time being.