With the Arrowverse's Supergirl series coming to a close after six seasons, the odds that a Supergirl movie set in the DCEU will be made have never been better. A solo film for the Girl of Steel had been discussed before but placed on hold due to various factors, including Warner Bros. policy at the time of not allowing multiple versions of the same superhero character to exist in different live-action properties.
Many had suspected that Supergirl season 6 might be the series' last, due to the pregnancy of star Melissa Benoist. The Supergirl star had already had a reduced role during the filming of the later half of season 5, and the premiere of season 6 was delayed to April 2021 to cover her maternity leave. With her contract only extended through the end of season 6, Benoist, the show's producers and The CW agreed to bring the show to an end, closing out another one of the establishing series of the Arrowverse.
With the Supergirl series ending, the way has been opened for a long-proposed Supergirl solo-film set in the DC Extended Universe. Such a movie had been in the works for years, but was recently confirmed to be on hold after reports that it could begin production later this year. No reason had been given for the delay, beyond one report that Warner Bros. was reconsidering what to do with Superman on a cinematic level. This in itself conflicts with reports that Henry Cavil is still signed on to play Superman in any upcoming DC Comics movies. In any case, it makes sense that Warner Bros. might not want to muddy the waters by introducing a new Supergirl film when it is unclear what further steps (if any) might be taken in developing the shared cinematic universe started by Man Of Steel.
Ironically, a solo Supergirl film might be one of the few properties Warner Bros. could safely produce while appeasing fans wishing to see a more interconnected cinematic universe and appealing to a broader audience. Zack Snyder lay the groundwork for introducing Supergirl in the original Man of Steel with an Easter egg suggesting that there was one Kryptonian living on Earth long before Kal-El arrived and was discovered by The Kents. He also wrote a Man of Steel tie-in comic establishing the DCEU version of Kara Zor-El existed and was part of the Explorer's Guild on Krypton. Whether or not she was the Kryptonian who crash-landed on Earth some 20,000 years before her cousin became Superman was never confirmed but the hints were there.
It would be a simple enough matter to build on Snyder's base idea and make a Supergirl movie based on Kara Zor-El's adventures before she left Krypton or at any point in history she watched the evolution of mankind. The film could use the same formula as the 2017 Wonder Woman movie, introducing Kara as an outsider forced into action in a conflict that is not her own, as she tries to find a place for herself in a new world. Such a movie could easily be introduced into the DCEU canon without upsetting the continuity of any films currently in production and, thanks to the TV series, there's a built-in audience of Supergirl fans who would love to see a story centered around Kara Zor-El as something other than Superman's cousin. Zack Snyder might even set it up with some of the new footage being shot for his cut of Justice League.