I honestly wish with my entire soul that joker and harley were just goofy partners in crime, not together romantically, not abusive in any way, just operating like a performance duo at a circus who live mostly separate lives after the show, harley with her girlfriend ivy, and joker pinning after batman, as it should be
TL;DR: Joker gets a nightmare about Edward and Harley having a child, Joker later in the day starts a fight with Edward. Poor Eddie's confused as hell while Jonathan and Harley are the same.
Based off these things from Arkham Knight where you explore Joker's fears.