Much like how lately Batman inspires Superman's costume, DC wants to make Harley a founding member of the BoP. Revisionist history is all the rage. As most suggested, Sirens would have been the way to go but I'm sure the powers that be shot it down since they live to put IP on hold for bigger projects and they had Catwoman coming in the Reeves film.
I thought the movie was fun, but yeah, it wasn't a BoP movie at all. It was a vanity project for Margot, but damn if it wasn't a good time.