It'd be nice, but I dunno how well it would work without addressing Superman's existence first - and after what WB allowed to be inflicted on Batgirl and Catwoman (and Supergirl previously) and that last pitch that went by for Zatanna, I must say I don't see them demonstrating a lot of respect for leading female comic-book characters who are tied indelibly to their lore.
For example, even outside of comic fandom, most adults who had the misfortune of walking relatively blindly (i.e. attached to their kids) into B&R probably remembered that Batgirl was Commissioner Gordon's daughter Barbara from the old '60s TV show. I'm only going to mention Catwoman by its name, since that's all there really is to connect it to the comics.
As for the previous Supergirl movie, its misfortune was being very poorly made; the Salkinds must have believed they could coast the rest of the way by that point. To their credit, the writers DID attempt to fit it into the existing Superman film continuity.
A new Supergirl movie is a nice idea, but I wouldn't hold my breath waiting for it unless Whedon does a half-decent job on WW. In fact, I'd like WB to focus their energy on big guns from DC who DON'T have a bat symbol or red 'S' on their chests.