Harley really isn't that complex. That isn't to say she isn't a good character.
And, truthfully, it doesn't matter what medium she was adapted from. The second Harley Quinn became a comic book character was the second Nolan was obligated to respect that. Jesus Christ. I mean, by your logic, the Spider-Man movies could turn Mary Jane into Firestar, and the X-Men franchise could turn Moira MacTaggert into X-23.
And it would be an unexpected twist, but the backlash would be so strong, that WB wouldn't dare try it. Besiades, it's a pretty stupid idea anyways.
On top of that, Dawes is too sane to ever try that, and way too good-hearted. Also, Holmes doesn't have the acting chops.