I think the best direction for her would be for her to become the Joker's successor. Like have her become so deranged and twisted she rivals even Joker. I think the best way for this would be for her to become like Gotham's crime Queen and I think she could do it she gets underestimated a lot but she's a psychiatrist and if she got her act together she could definately be a very formidable enemy for Batman. I'd like her to maybe betray Joker to Batman and piss Joker off and after he escapes she becomes terrified of him to the point she starts obsessing over gaining more power and starts forming alliances and ultimately surprises every criminal in Gotham with how manipulative and smart she truly is and even giving Batman a run for his money and I like the idea of her being the first super villain to really force the Bat Family of Gotham if only for a short while like she uses her psych degree to profile each hero in Gotham and systematically takes Batman and his allies down.

Then Harley finally finds Joker and chaos ensues Batman and his allies show up and Joker is revealed to have been watching her and she assumes he was going to kill her and instead surprises her by expressing pride in her. I like this idea because it makes sense Joker has shown a lot of jealousy when Harley outshines him but I wouldn't be surprised if he saw her rise to power and do what he couldn't (like temporarily drive Batman out of Gotham for example)and it would make sense if he coped with that by beginning to view her as an extension of himself I mean she's his creation after all and in that wouldn't be surprising if in that regard Joker and Harley ultimately had a moment like this.