Spoilers ahead for Justice League. Considering that DC rolled out ComicCon promo material back in July teasing the idea of a Green Lantern cameo in Justice League, fans might be a little let down that Ryan Reynolds doesn't make an official suited-up appearance in the new film. However, that doesn't mean the movie has no Green Lanterns in it. We may not get to see Reynolds' particular Lantern in the film, but a Green Lantern's Justice League cameo definitely happens, and it may hint at the torchbearer's future in the DC Extended Universe.
In the DC Universe, Green Lantern is just one member of a universe-wide intergalactic police force watching out for trouble, with each individual Lantern responsible for patrolling their particular section of the galaxy. Each Lantern bears a Green Power Ring, their symbol and source of power. Forged from an ancient and magic energy source, these rings seek out and bond to individuals with great willpower who are able to overcome their fears.
In Justice League, meanwhile, alien demi-god Steppenwolf attacks Earth, intent on finding a mysterious object called a Mother Box. When Bruce Wayne puts the call out for other metahumans to help battle, Diana Prince is one of the first he turns to, and the hero otherwise known as Wonder Woman is happy to help. She tells Batman that this isn't the first time Steppenwolf has tried to take over Earth. In a flashback scene, she describes how there was a war against Steppenwolf when he first came thousands of years ago. Atlanteans, Greek gods, and the Amazons united along with others in an all-out battle to defeat the immensely powerful Apocolyptian — and Green Lantern was there, too.
During this scene, there's a brief but noted appearance of an alien Green Lantern fighting alongside the others. It's impossible to mistake the character for anything else, as Steppenwolf takes notice and pauses to watch a Green Power Ring float away into the sky. In the lore of Green Lantern, this means the current Green Lantern has died, and the ring is seeking the next ring recipient to become the Green Lantern.
That brief appearance, untethered from the 2011 version of Green Lantern, actually points to the future of the character (or should we say... characters) in the DC Extended Universe. Currently slated for 2020, DC's Green Lantern Corps is a movie focusing not on an individual Green Lantern, but the larger galaxy-guarding group. ScreenRant noted that Warner Bros. has confirmed the appearance of two different Earth Green Lanterns, though thee studio has said little else about the film, not even officially confirming that David S. Goyer (The Dark Knight Trilogy, Batman V Superman) is working on the script. For now all we know is Hal Jordan, who was last seen in 2011's film and was the first Extended Universe canon Green Lantern, is definitely in, as is John Stewart, the backup for backup Lantern Guy Gardner. After Gardner got hit by a car, Stewart was next in line for the ring when Jordan stepped back from the role. Stewart is also notable for being DC Comics' first black superhero.
Speaking of comics, Green Lantern has been a notable member of a certain League affiliated with Justice all through the Silver and Bronze comic eras. While Batman and Superman rarely featured in main storylines, Green Lantern was one of the comic Justice League's main five, consistently appearing and contributing to major plots. Whether Green Lantern Corps will have Jordan or Stewart joining the team remains to be seen, but for now, Justice League points the way to a greener future.