Hawkwoman is my first pick for the movie (especially since you'll have some fans expecting her anyways), but I also like the ideas of Zatanna or Vixen since they have unique powers. I think Zatanna would offer a better look into the magic world of DC than Shazam. Also, I agree about the comic book logic thing with magic vs science. That's the aspect of the MCU Asgardians as aliens I didn't like.
It's important to look at the numbers regarding this escalated backlash. I mean, you have people angry on Twitter/Tumblr (no surprise there), but has it really seeped into any other news outlet. I'm not talking about any outlet covering the fact that Whedon left Twitter, but are any of them actually talking about anything in AoU that could be perceived as sexist? I think this is fairly isolated.