A potential Man of Steel 2 could expand the mythology of the Last Son of Krypton, with Henry Cavill returning as Superman. Before the news of his return, Clark Kent's future on film was questionable, and it seemed at one point he might have been replaced by his cousin. That's likely no longer the case, but Sasha Calle's Supergirl is still being introduced in the upcoming The Flash. Given the plans for that character at one point, she'll likely remain a firm fixture in the DC Universe. This could combine the new theatrical Supergirl with an iconic yet alternate take on the character from the comics -- Powergirl.
The Flash Will Introduce Supergirl to the DCU
One of the elements of The Flash that many fans are looking forward to is Calle's Supergirl. Seemingly combining Kara Zor-El with the Flashpoint version of Superman, this character could exist in a separate timeline from the DCU that The Flash explores. Nevertheless, the rumored idea of Calle's Supergirl being taken from her timeline and trapped in the DCU could remain, allowing Cavill's Superman to stay in the spotlight while bringing in his cousin.
This sets up the potential for Power Girl. In the pre-Crisis on Infinite Earths continuity, Power Girl is the cousin of Superman from another timeline. Stuck on the "Prime Earth," she's treated as an alternate Supergirl. In her own reality, she came to Earth as a young lady, being raised by Clark and his wife, Lois. Not wanting to be seen as a female Superman, she took the identity of Power Girl. Such an origin could easily be used for Calle's Supergirl going forward.
Now being part of the DC Universe's history would put her in a position to be taken in by Lois and Clark, who try to acclimate her to this new Earth if she's from another universe. As she comes into her own and becomes a more fully-fledged heroine, she could even forge her own path with the "Power Girl" identity. After all, Calle's hair in images and footage for The Flash resembles Power Girl's signature bob haircut, albeit black instead of blonde. As much as this would make sense, there are other ways in which Powergirl could be introduced.
The DCU Might Make Power Girl Separate from Supergirl
Both Supergirl and Power Girl had various origins after Crisis on Infinite Earths in an editorial effort to keep Superman as the Last Son of Krypton. These were eventually done away with in order to get back to the more classic takes on the characters, but these alternate takes offer ways to keep Supergirl and Power Girl separate. One would be Power Girl's Atlantean origin, where she was a powerful descendant of the Atlantean wizard Arion. This gave her all manner of powers, which coincidentally mirrored those of Superman. Though the origin is now no longer canon, adapting it to the DCU would allow Power Girl to be separate from Supergirl fully.
Another "version" of the character debuted in the cartoon Justice League Unlimited in the form of Galatea. A clone of Supergirl developed by Project Cadmus, this woman had a costume and haircut resembling Power Girl, even wearing a red towel like a cape at one point. Such a storyline could be used in the films, with scientists trying to clone Supergirl and producing an older version of her in the process. From there, the initial antagonist, Galatea, could become a hero in her own right, becoming the DCU version of Power Girl. Such a direction would keep her tied to Supergirl without needing to resort to multiverse shenanigans.
It would also utilize the "clone" storyline in a new way that doesn't involve the villainous Bizarro or Clark's younger clone, Superboy. Drawing from these different versions of Supergirl and combing them presents all kinds of character possibilities, expanding the Superman Family while putting both Power Girl and Supergirl on the big screen.