Ever since the release of Justice League, Warner Bros. so-called DC Extended Universe has been going through an overhaul. After Man of Steel, Batman v. Superman, Suicide Squad, Wonder Woman and the superhero team-up film, the studio is now looking to produce films outside of DCEU continuity. The first of those movies, a Joker origin movie, is already in the works. There have also been other reports of various films that will take place outside of DC's main cinematic continuity, which will free up creators to tell different stories without being held back by other films.
But that doesn't meant that the DCEU is on its way out. Before the year is over, fans will get to see James Wan's Aquaman, while 2019 includes the highly-anticipated releases of Shazam! and Wonder Woman 1984. And, in addition to a slew of Bat-family films in various states of development, an the hope for a new Superman film, comes the announcement that a Supergirl film is in the works.
What we don't know yet, is if Supergirl is supposed to be separate from the mainstream continuity, like the Joker origin movie, or if it will be a part of the DCEU. While the firmer is possible, the latter makes more sense. In fact, there's a solid case to be made for Supergirl to be a direct spin-off from Zack Snyder's 2013 film, Man of Steel.
The Man of Steel Connection
Ever since its release, fans have believed that Man of Steel hid a sublte Easter egg indicating Kara Zor-El's existence on Earth in the form of an open pod that could be seen in Clark Kent's Kryptonian spaceship. This belief was only made stronger by a Wal-Mart exclusive comic book meant to be a tie-in to the movie, which made it clear Kara was on the Kryptonian ship thousands of years ago, and was now somewhere in the DCEU. For most fans, introducing Kara Zor-El would just be a pay off of this Easter egg, long ago established.
However, the comic book wasn't truly official canon. In fact, Zack Snyder was recently asked about that very pod, and the director answered that it was meant to serve another purpose, one unrelated to Supergirl. Which, really, might be for the best, because now Supergirl can tell an origin story that is much closer to the comic books.
Man of Steel went to great lengths to introduce a fleshed-out Krypton, and we don't need a separate movie introducing yet another version of that mythos. Instead, Supergirl should be allowed to build from what's been established. The film could show its very own flashbacks on the doomed planet, and see its destruction from the point of view of the young Kryptonian girl who would find herself on Earth years later. What's more, it could also draw some very important comparisons with another Kryptonian audiences are familiar with.