Warning: mild SPOILERS for Justice League are ahead!
The Justice League is currently made up of six members in the DC Extended Universe, but just like in the main DC Comics mythology, there are others out there fighting evil. From Batgirl and Nightwing to Green Lantern and Shazam, there are many superheroes who are slated to debut in the DCEU over the coming years, but the Justice League movie snuck in a fittingly small Easter Egg for a benevolent character who hasn't been announced to appear in this franchise. During one of the moments spotlighting Cyborg, there's a quick reference to Ryan Choi, who's better known to DC Comics fans as the modern day Atom.
Relatively early into Justice League, video footage was shown of Silas Stone trying to bring his son Victor back to life at S.T.A.R. Labs. As we all know, the end result was Dr. Stone using a Mother Box to rebuild Victor's destroyed body, which resulted in him becoming Cyborg. But now one eagle-eyed Redditor noticed that one of the names presented on the side of the screen during the footage is Ryan Choi, indicating that he is a S.T.A.R. Labs employee in the DCEU. Whether Ryan will ever appear in person in this universe, let alone become The Atom, remains uncertain, but at least it's confirmed he's putting his scientific expertise to good use in this world.
Ryan Choi was introduced in 2006's DCU: Brave New World as the protege of Ray Palmer, the original Atom. When Ray disappeared, Ryan inherited the Atom mantle and fought evildoers as the shrinking superhero until he was killed by Deathstroke. Ryan was reintroduced in the DC Rebirth relaunch last year, where, like in the previous continuity, he became The Atom after Ray went missing (someone really needs to put a tracker on him) and is currently a member of the Justice League of America. Outside of the comics, Ryan's most prominent appearance has been in the Batman: The Brave and the Bold animated series, and he will also be a downloadable character in Injustice 2.
At the end of Justice League, Batman decided to convert the fire-damaged Wayne Manor into the superhero team's headquarters, and Wonder Woman suggested they leave open spots at the meeting table for others to join their cause. I suspect folks like Green Lantern and Martian Manhunter will be given priority for membership if/when Justice League 2 happens, but with Brandon Routh's Ray Palmer currently serving as The Atom in The CW's DC TV universe, it would be cool if the DCEU cast someone to Ryan Choi and have him serving as the size-altering crimefighter.