Also, the problem with a Supergirl movie is that EVERY Supergirl origin is either ridiculously lame or diminishes Superman's 'Last son of Krypton' schtick.
A very fair point. So, why not make her a parallel universe female Kal-El? Jor-El had a daughter instead of a son, and launched her. Ergo, Supergirl, with a non-lame origin that doesn't diminish Clark's.
Then you'd have Power Girl.
I dunno...I think the "another planet from another universe" aspect would be too much for a mainstream movie.