^^ Er, no; Power Girl is an alternate version
of Kara/Supergirl, who has AFAIK always existed alongside Superman in some form, whereas I'm proposing replacing Superman for this particular movie. An entirely different kettle of fish. As for the alternate universe angle, so long as there weren't any crossovers to Kal-El's universe, I hardly think audiences would be confused, seeing as how they embraced the loopier Trek XI
without any trouble. It'd just be a Superman story, maybe showing Ma and Pa Kent, but with a chick. No problemo.
I know what and who Power Girl is. In short, she's Supergirl from an alternate universe.
What I didn't understand is that you meant the whole movie would take place in the alt-universe, instead of having Supergirl cross-over to "our" universe. I am still not sure how well that would fly (no pun intended). I think audiences would go "Where's Superman? How come they didn't at least mention Superman?"
How would it be addressed that this was an alternate universe w/o the film addressing it as such or some sort of cross over? Otherwise it would just look like "Hey, we made a Superman movie, but made Superman female!"
Personally, if they ever wanted to make a Supergirl movie, I would suspect the most promising way to do it is to introduce her in a Superman movie and then have a spin-off (kind of like how they crafted the Electra movie - only hopefully with a higher quality product).
Speaking of which, I always wanted to see a Smallville episode with this sort of conceit, in which Clark somehow enters an alternative reality where a daughter of Jor-El and Lara came to Earth. Clark and his "sister" could then kick some butt together before the former went home.
That would probably be the best way to introduce Power Girl on Smallville if they wanted to.