At this point if there's any supporting character who could carry their own film, it's Harley. Her popularity is through the roof now (even her comic is outselling the main Batman title).
Rather than an outright Bonnie & Clyde inspired film, I'd probably so more of an ensemble picture where they headline. Sprinkle in a bit of Gotham Central, Sirens, and Birds of Prey. The Bat-universe is vast, they can really take advantage of the lower-level echelon of crime. No large-scale threats or destruction. Just a really fun and violent crime thriller romance.