The recently released trailer for Zack Snyder's Justice League revealed a bunch of new scenes in the upcoming film, including shots of Darkseid, DeSaad and Superman's black suit. Overall, the trailer promises a lot of new scenes in Justice League, but one in particular could be more significant than the others, as it might just introduce Supergirl to the DC Extended Universe.
At the 0:24 second mark of the trailer, there's a shot of Ben Affleck's Bruce Wayne looking at a holograph of what seems to be a Kryptonian, which matches the 2017 trailer. The shot itself sees Bruce pensively staring at an unknown person in a hologram while mentioning a lack of Kryptonians left to defend Earth. There were a lot of fan theories about this originally, as there are some clues (including body shape and tattoos) that this isn't Superman (Henry Cavill). Instead, this may actually be Kara Zor-El, Superman's cousin. The fact Snyder brought this image back for the trailer may be hinting at her introduction in the future.
Notably, a DCEU comic, 2013's Man of Steel: The Prequel, introduced Kara as a starship pilot who crashed on a foreign planet after her crew got attacked by an escaped Kryptonian criminal. This caused Kara to land on Earth, as the navigational system on her ship was fried. And with the story occurring thousands of years before Man of Steel, many wondered if she was alive. Fans even felt the open pod Superman ventured into was hers, although Snyder poured cold water on that notion and said Bruce was looking at Cyborg projecting Superman.
But that could have been misdirection, as Snyder did admit he wanted to build a world for Supergirl. When pressed numerous times on social media on the new cut, he did say "stay tuned," so it's possible there's more with this shot. Whether Kara is frozen in the Arctic or she actually fought on Earth in the past and retired, Synder can find a way to bring her back. This scene can also easily be retconned so that Bruce took data from one of the downed Kryptonian ships as he was searching for warriors and this hologram showed Kara.
She could even be in a stasis chamber somewhere, or maybe she ended up trapped in the Phantom Zone, as Man of Steel did make clear Kryptonians can survive there. Ultimately, though, Snyder would likely only include pertinent footage in his cut and wow-factor content that would have expanded his DCEU vision. He's talked about things like Martian Manhunter being included, as well as Ryan Choi, so there's no reason Kara couldn't also make an appearance in the future.
Justice League stars Ben Affleck as Batman, Gal Gadot as Wonder Woman, Henry Cavill as Superman, Amy Adams as Lois Lane, Jason Momoa as Aquaman, Ezra Miller as The Flash, Ray Fisher as Cyborg, Jeremy Irons as Alfred Pennyworth, Diane Lane as Martha Kent, Ciarán Hinds as Steppenwolf, Jesse Eisenberg as Lex Luthor and J.K. Simmons as Commissioner Gordon. Zack Snyder's Justice League will premiere exclusively on HBO Max in 2021.