To modify my story idea even further, Harley doesn't actually KILL Joker. She imprisons him, a la Kathy Bates in Misery, and THEN takes on his persona, making people believe she killed him. The hook on the story is that Batman finds out that Joker is still alive, and has to wrestle with whether to rescue him. Eventually, he does, and he and Joker team-up to take down Harley, who has become an epic menace as bad, if not worse, than Joker.  While Joker appreciates and respects what she's done