Eh, this is an interesting idea to me and I wouldn't mind it with my personal canon Hawke. I played her as amoral, devoted to her family (and love interest) above all else, and prone to jumping into fights. So I could actually see her making sense as an antagonist, particularly since she was never exactly heroic, and particularly since she romanced Anders and would likely attack anyone that seemed to be a threat to him.
That being said, not everyone has a Hawke like my Hawke (and in fact, I wouldn't be surprised if my version of Hawke is atypical). The "villain" idea works for her because of how I played her and see the character, but it would make a lot less sense for someone that played Hawke as super squeaky clean or diplomatic. So while i personally find the idea interesting, I don't think I'd actually like to see it. I just can't see how it could work.