You can start with the right makeup. Focus on bold colors for the eyes and lips. Also, get a similar hairstyle, like the short and wild one Harley Quinn has.
It involves getting the right makeup, costume, and hairstyle. The makeup should have bold colors and unique patterns. The costume needs to be accurate in design and fabric. And the hairstyle should be wild and distinctive.
Harley Quinn's character can be developed by showing her struggle with her identity after the Joker's death. In Arkham Knight fanfiction, she could start to question her loyalty to the Joker's legacy. Maybe she begins to form her own moral code, separate from the chaos the Joker brought.
One common theme is Harley Quinn's relationship with the Joker. Even in the Arkham Knight setting, her past with him often plays a big role in the fanfictions. Another theme is her interactions with Batman. Since they are adversaries in the game, fanfictions might explore different aspects of their confrontations or even rare moments of cooperation.
A great plot idea for this fanfiction could be that Harley Quinn is haunted by the memories of her time with the Joker during the Arkham Knight scenario. Batman, noticing her distress, tries to help her. As they interact more, Harley starts to see Batman in a new light and decides to assist him in his fight against the Arkham Knight. Along the way, they face many challenges, like dealing with Harley's old gang who don't trust her new alliance with Batman, and also the Arkham Knight's devious traps. This would make for a complex and engaging fanfiction story.
Well, another place could be some dedicated Batman or Harley Quinn fan forums. Sometimes, fans share their own written fanfictions there. Reddit also has some relevant sub -reddits where people might discuss or even post their own Harley Quinn fanfictions in the context of Batman Arkham Knight. You have to dig a bit, but it can be rewarding.