He could be a henchman who has a change of heart. Maybe he starts off working for one of Harley's enemies but realizes that Harley's brand of chaos is more appealing. He could then become her ally, helping her with her schemes and providing muscle when needed.
If the male OC is a journalist, he could be trying to expose Harley's story to the world. But as he digs deeper, he gets caught up in her world and starts to see her as more than just a crazy criminal. He might end up falling in love with her or at least becoming a very close friend, which would change his perspective on the whole Gotham underworld.
One great 'oc x Harley Quinn fanfiction' is 'Harley's New Ally'. In this story, the original character (oc) is a vigilante who crosses paths with Harley Quinn. They start off as enemies but gradually develop an understanding. The author does a great job of capturing Harley's complex personality and how the oc challenges her views. It has some really interesting action scenes and character interactions that make it a fun read.
One common theme is redemption. Often, the OC might be involved in trying to help Harley find a new path, away from her villainous ways. For example, in some fics, the OC could be a hero - type character who believes in Harley's potential for good.
Joker would probably see the Twilight world as a new playground. He might try to mess with the Cullens, seeing them as too 'perfect' and in need of some chaos. Harley, on the other hand, could be fascinated by the idea of eternal love in the Twilight series. She might even try to get involved in the relationships, perhaps causing some jealousy or misunderstandings.
Power dynamics is another theme. Given Harley Quinn's complex relationship with power, especially in relation to the Joker, the male oc can bring a new element to this. Maybe he has his own power struggle within the DC underworld, and his relationship with Harley Quinn either complicates or simplifies that struggle. It could also involve themes of loyalty and betrayal, as in the world of DC, alliances can be fickle.
One possible title could be 'The Adventures of Harley Quinn and Her Fem OC Ally'.
A good 'oc x Harley Quinn fanfiction' often has unique plot twists. Maybe the oc has a secret that changes Harley's perception of them completely. Or perhaps Harley has a goal that the oc initially opposes but later comes to support. The story should also have good writing in terms of grammar and style. If the writing is sloppy, it can be really hard to get into the story. And the fanfiction should explore the themes that are relevant to Harley Quinn's character, like freedom, madness, and the search for identity, through the relationship with the oc.
Give the OC a unique backstory. For example, if they grew up in the slums of Gotham but had a love for art, it could influence their actions and interactions with Harley. Maybe they use their art skills to create elaborate disguises or distractions during their escapades.
One interesting aspect of 'Batman and Harley Quinn Love Fanfiction' is how it reimagines their relationship. In some fanfictions, Harley Quinn's complex character is softened by Batman's influence. It's a unique take as in the original comics their relationship is more of an adversarial one. Fanfiction allows for a different exploration of their personalities and how they could potentially interact on a more emotional and romantic level.
One common theme is friendship. Often, the female oc starts as a stranger to Harley Quinn and they gradually become friends. They support each other through various challenges in the Gotham underworld.
One good 'Joker Harley Quinn OC fanfiction' could be one where the OC is a new villain in Gotham who tries to disrupt the relationship between Joker and Harley Quinn. Maybe they start by causing misunderstandings between them, like forging messages from Harley to Joker saying she's leaving him for someone else. This could lead to a whole new dynamic where Joker is on a rampage looking for Harley, while Harley is confused as to why Joker suddenly turned on her.