Zack Snyder's Justice League will repeat Man of Steel's Supergirl Easter egg. Snyder is responsible for launching the DC Extended Universe in 2013 with the release of Henry Cavill's first Superman solo movie. Man of Steel was the start of what Snyder imagined as a five-film plan focused on the Kryptonian hero, but those plans changed and expanded as the DCEU tried to compete with the Marvel Cinematic Universe. Although these plans grew beyond Snyder, he is responsible for introducing Superman, Batman, Wonder Woman, Aquaman, Cyborg, and Flash.
With his knack for worldbuilding and details, Snyder also planned to incorporate many other DC characters through his stories. The Snyder Cut of Justice League will include some new ones to prepare the world for an eventual battle with Darkseid, but Snyder also planted seeds for other heroes to emerge. One of these came in Man of Steel when Kal-El visited the Kryptonian Scout Ship and fans spotted one sleeping chamber was mysteriously opened. This was meant to be a setup for Kara Zor-El but Snyder never got a chance to pay off the tease.
Following the release of the updated Justice League trailer, Snyder hosted a Q&A on Vero about his version of the movie. Several topics were discussed as Snyder broke down the footage and answered fan questions. One of them asked how many Kryptonians are on Earth besides Kal-El. The question indirectly references the possibility of Supergirl being around on Earth and even being teased in Justice League. As it turns out, Snyder confirmed that Justice League will once again include the open sleeping chamber tease from Man of Steel but that the answer to what happened is a story for another time.
Well, the easy answer is only Kal-El. But, there's always... that's kind of an open-ended... If you study Man of Steel, and even in this film, there's a reference to the open sleeping creche. Which is when they walk by it on the scout ship when they're bringing Superman's body in to bring it back to life, there's a sequence where they go kind of walking through that same hallway where Kal-El walked through when he first came into the Scout Ship. Then there's a moment where he kind of wipes the dust from a Kryptonian sleeping creches and sees this sort of shriveled Kryptonian inside. Now, next to that creche there's an open creche that just looks like someoene clearly got out of that and who that is remains a mystery to this day. We'll see.
Whether or not Supergirl is referenced in Justice League beyond the open pod, Snyder has already had the chance to explore some of Kara's story. He wrote a Man of Steel prequel comic book that explored Supergirl's origins. The story focused on Kara's days as a member of the Explorer's Guild, how the Scout Ship crashed on Earth, and confirmed she was the lone survivor. Snyder has gone on to discuss that the open pod was set up for much more than just Supergirl, though, but it has yet to be revealed what that means.
There isn't an expectation at the moment that Supergirl will appear in Justice League or if she plays a role in any future Justice League stories Snyder already has planned. However, Snyder's already proven he's full of surprises, such as bringing back Jared Leto's Joker for a brand new scene. That might not happen with Supergirl in Justice League since she was never cast, but it wouldn't be the craziest thing to happen with the project. Any version of Kara that is featured in Snyder's DCEU in the immediate or distant future will be quite different than the one most comic fans are familiar with. But, it would be intriguing to see how she fits into a post-Justice League storyline, especially if the Knightmare future comes to fruition and Superman turns evil.
Source: Zack Snyder