As Wonder Woman continues to generate excitement for the DC Extended Universe, it's only natural that fans' thoughts should turn to another leading lady: Supergirl. Specifically, how DC Films could use the soon-to-be-resurrected Superman to introduce her into the movie universe. Where Batman V Superman was originally conceived as a direct sequel to Man of Steel, it's now clear how much it was changed into a gateway for introducing Batman, Wonder Woman, and the Justice League as a whole. Therefore, the prospects of a true sequel to Man of Steel continue to generate rumors and wishful thinking: Black Adam's debut as the villain and Matthew Vaughn directing Man of Steel 2 are just a few.
More recently, a new rumor suggested that Man of Steel 2 would introduce Supergirl to the DCEU... and faster than a speeding bullet, that rumor was debunked. However, the actual plans for Man of Steel 2 are still in the earliest stages of DC Films' Kryptonian birthing matrix, meaning the Girl of Steel joining her cousin in the DCEU remains a possibility. (Fans are even designing what Supergirl's DCEU suit would look like.)
Now there is the matter that Kara Zor-El has already arrived on Earth to be dealt with, as depicted in the 2013 comic book one-shot Man of Steel Prequel: Special Edition, chronicling Kara's journey to Earth thousands of years in the past. We've explained how that twist makes a Supergirl cameo in Justice League possible, but if the events in that little known comic book remain canon, it would be a serious waste of one of the most famous female superheroes in the world.
It seems just as likely, if not more, that it will be justifiably ignored as plans for a new Supergirl in the resurgent DCEU take shape. And here's one way to pull it off.
HOW SUPERGIRL CAN JOIN MAN OF STEEL 2
The most likely scenario for introducing Supergirl, and the one likely to please fans responding to the traditionally heroic version of Wonder Woman's origin Patty Jenkins delivered, is to preserve Supergirl's classic origin. Rocketed from the dying Krypton to Earth at the same time baby Kal-El was, and responsible for raising her infant cousin on their adopted world. But something went terribly awry for Kara during her journey to Earth (on the TV show, her ship fell into the Phantom Zone), and she arrived decades after Kal-El. By the time Kara reaches Earth, Kal-El has already matured into an adult and become Superman.
There are a few excellent depictions of Supergirl arriving on Earth in the last decade and a half of DC Comics, with the popular 2004 story in Jeph Loeb's Superman/Batman series among the best known. Titled "The Supergirl from Krypton," the origin was a successful reintroduction of Kara Zor-El to DC Comics continuity (later adapted into the animated film Superman/Batman: Apocalypse). It begins when Batman and Superman investigate the crash landing of Kara's ship in Gotham, after which Clark learns he has a cousin who survived his homeworld's destruction.
Clark briefly tries to mentor Kara into Earth life but she is soon taken in by Wonder Woman to live and train with the Amazons on Themyscira. Kara is then abducted and corrupted by Darkseid of Apokolips - the big bad whose lieutenant Steppenwolf will fight the Justice League - and briefly becomes evil before she's saved and turned back to her true, good self.
All the elements of that story already exist in the DCEU post-Justice League, and would offer a chance to both build onto the DCEU, and return to one of the fandom's new favorite locales. Not to mention give the DCEU's Superman exactly what he's been missing...