Harley's character could develop in many ways. She might become more protective and motherly, which is a big change from her usually wild and carefree self. This new - found sense of responsibility could make her question her relationship with Joker and the life of crime they lead.
One idea could be that Harley tries to hide her pregnancy from Joker at first because she's not sure how he'll react. Another could be that they go on a wild adventure while Harley is pregnant, facing various villains who either try to use her condition against them or are unexpectedly helpful. Or perhaps there's a plot where Harley has to deal with her own insecurities about becoming a mother while still being in a relationship with the unpredictable Joker.
One interesting plot could be the chaos that ensues when Harley discovers she's pregnant. Maybe she tries to reform Joker into a more family - friendly person. Another could be the reaction of Gotham's villains and heroes to the news. For example, Batman might be conflicted on how to deal with a pregnant Harley who is still associated with the Joker.
Their relationship can become more tender. Harley might see Joker in a new light as he shows some concern for her and the unborn baby. For example, he might start to protect her more from other villains.
The Joker might react with anger. He could see the pregnancy as something that will tie him down and interfere with his criminal activities. He's not exactly the family - minded type, so he might try to force Harley to get rid of the baby.
Well, the plot could involve Harley Quinn and the Joker having an unexpected pregnancy. It could show how their relationship changes. The Joker might initially be dismissive or even angry, thinking it'll disrupt his plans. But Harley could be determined to have the baby and try to make the Joker understand the importance of family, or at least her version of it. There could also be a lot of drama with other villains or heroes reacting to the news of Harley being pregnant.
Their relationship could develop through shared experiences. For example, they could go on heists together and during those times, Joker could start to see Harley in a different light. He might notice her intelligence and loyalty more and more.
One common plot twist is that the Joker might initially reject the idea of the baby but then have a change of heart later on. Another could be that Harley Quinn discovers something sinister about the Joker's intentions regarding the pregnancy, like he has an ulterior motive that's not about building a family. Or perhaps, Harley Quinn loses the baby in a tragic way which then sends her on a vengeful or self - destructive path.
One great 'Joker OC Harley fanfiction' could be 'The New Face in Gotham'. It's about a new original character who gets involved with the Joker and Harley's chaotic world. The story has great character development for the OC and shows how they interact with the iconic duo.