The important thing here, as Whedon says, is figuring out his version of the character and casting from there.
When he was further pressed about the idea of not casting a known name, Whedon believes the Batgirl name will carry more importance than the person behind the mask.
Figuring out how he will tell this story and what he will require from his lead should come first, and thankfully it sounds like Whedon is going about the Batgirl movie in exactly that way.
I’m not against movie stars, they’re great, but you need somebody who’s going to be just right.