The costume that Sasha Calle's Supergirl dons in Andy Muschietti's Justice League.
Within DCEU canon, Superman's suit wasn't designed to be a superhero costume. Superman's DCEU Kryptonian suits are sleek, flexible, and incredibly durable. They're also known for their lack of a belt and the absence of Superman's infamous "external underwear." They're metallic-looking, form-fitting, and decorated with subtle angular shapes.
Sasha Calle's short black hair on the set of The Flash suggests she's playing a new version of Supergirl — possibly Lara Lane-Kent, a Supergirl from a dream world featured in the Injustice comics and video games, where she is Superman's daughter instead of his cousin, Kara Zor-El. Although not as shiny as Superman's, Supergirl's costume in The Flash shares the same patterns, the same angular shapes, and the exact same chest insignia as the DCEU's Kal-El, as well as also the same amount of red on the shoulders that Lara Lane-Kent usually sports. As Supergirl's live-action suit resembles Superman's so much, it's likely that Henry Cavill's Kal-El will play an off-screen role in her origin story, to some degree.
The presence of Michael Keaton's Batman proves that The Flash has the freedom to feature almost any alternate version of famous DC characters. Supergirl could be revealed to come from an alternate dimension where Superman never existed, and the concept of the multiverse would justify it. With Melissa Benoist's depiction of Kara Zor-El in the Arrowverse coming to an end after six seasons and a Superman reboot in the works, it's logical for Warner Bros. to reintroduce famous characters to the big screen but also take them in new directions.
Supergirl's DCEU costume is not the only surprise The Flash may have for fans. Barry Allen's Flash suit is also getting an upgrade, and fans of Tim Burton's Batman movies will be delighted in seeing a modern take on the classic 1989 all-black Batman suit. Whether characters like Henry Cavill and Brandon Routh's versions of Superman make surprise cameos is still a mystery, but it's within the realm of possibility.
