An Easter egg from the Justice League movie seems to indicate that the Atom, and specifically the Ryan Choi iteration of the character, exists in the DC Extended Universe.
The Easter egg, which can be viewed here, is quick and difficult to spot. As a computer lists the name of S.T.A.R. Labs employees, the name “Choi, Ryan” is visible.
The Atom is a more obscure set of characters in DC Comics’ large and impressive roster; ignoring the unrelated Golden Age version, the two main heroes to take this name are Ray Palmer and Ryan Choi, each of whom have a suit with the ability to shrink them down to microscopic size. Palmer invented this technology, while Choi took it up after Palmer went missing into other dimensions. (Comic books, right?)
Ray Palmer is probably the more famous version of the Atom, particularly since he appears in Legends of Tomorrow, a series in the popular “Arrowverse” franchise of TV shows. Until now, Choi has been absent from adaptations, save the Batman: The Brave and the Bold cartoon.
However, he has proven to be a popular character, and after controversially killing him off in the comics, DC Comics responded to criticism by reviving him and now has the two working together.
Could this indicate a plan to bring the Atom into the DCEU roster of superheroes sometime in the future? To be fair, Justice League is full of references, so perhaps this was just inserted as a quick wink at the fans. Then again, the LexCorp Tower and Wayne Enterprises satellites in Man of Steel turned out to be more than just Easter eggs, so who can say that Choi will not show up to save the day in a later film?