He certainly wasn't tied down with his interpretation of Selina Kyle (and neither was Tim Burton, who probably took the most liberties with that character, making her supernatural and all). Case in point, he could've used Andrea Beaumont. She shares the closest character resemblance with Rachel Dawes, as she was someone Bruce grew up with more or less (teenager to adulthood) and she was also someone he would've left his Batman career for (and almost did). Nolan wouldn't have to include her alto ego as the Phantasm, but possibly hinting at such a path, as well as including Beaumont's backstory and her connection with Bruce, would've been loads more interesting than creating a character out of the sky blue.