I want a Superman movie that introduces Supergirl. But I could see how they could do a movie with Kara and Kal-El, but without Superman.
If the first third of the movie was set on Krypton and Kara Zor-El is a litte girl and Kal-El is a newborn--then they would both be on Krypton around when Brainiac attacks Kandor. The rest of the movie would have young Kal-El and Kara Zor-El both on Earth. But they are separated and she becomes Supergirl and battles Brainiac. However by the end of the picture, Supergirl would find Kal-El now Clark Kent living with the Kents on their farm.
I know that Hollywood would like to cast some sexy adult actress as Kara Zor-El, but I think it would be better if Supergirl was much younger. That would help distinguish her from the TV show Supergirl and it would give them the kid angle, like with SHAZAM!
The movie could also operate as a backdoor reboot for Superman as they would change some of his early origin story and put new actors in the place of Jor-El, Lara Lor-Van, Jonathan Kent and Martha Clark Kent.