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Would you look at that, the OTHER female-centered superhero movie that didn't understand what it was doing. Though not as blatantly offensive as Catwoman, the Supergirl film still manages to take a character with a respectable history and turn her into a man-needing-witch-fighting-girl-school-attending goofball. Supergirl tried to appeal to fans by giving them some references to the mythology, but overall the final product isn't what fans would want. Plus, in the age of incredible special effects, this movie just doesn't do it anymore.
With the upcoming Man of Steel, we have reason to believe that Warner Bros. will attempt to emulate Marvel Studios' "larger universe" plan and while a feature based on Supergirl wouldn't be a top priority, what's to stop her from getting a film in the future? But how do you differentiate Supergirl from Superman in a big budget film? One way is to take the approach used in DC's "The New 52" wherein Kara's origin was altered to her being transported to Earth as well. Kara wakes up in Russia and believes only 3 days have passed, remembering her baby cousin Kal-El. Upon a fight with some armored men, she meets her fully grown cousin, now Superman. This idea offers a lot of room for expansion and unique storytelling, especially once audiences warm up to Henry Cavill's Man of Steel.
Likelihood of happening: If Man of Steel is a huge success, who knows? Seriously though, don't hold your breath.