One challenge would be acceptance from their respective communities. The heroes might not accept Nightwing being involved with Harley, a former villain. And the villains might see Harley as a traitor for being with a hero. Another challenge could be protecting the pregnancy from their enemies who might want to harm them or use the baby for their own gain.
Well, the plot could involve a strange experiment that causes Harley Quinn to become pregnant. Nightwing gets involved because he's trying to figure out what's going on. As the story progresses, they have to deal with villains who want to use the baby for their own evil purposes. They might also have to deal with their own personal issues, like Harley's past relationship with the Joker and Nightwing's loyalty to Batman.
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.
Maybe Harley Quinn discovers some dark secret about Nightwing's past that she uses to blackmail him into helping her with a heist. But as they work together, they start to understand each other better. Nightwing realizes Harley isn't all bad and Harley sees that Nightwing has his own demons. In the end, they might even become friends or something more.
First, understand the characters well. Harley Quinn is complex, with her history of abuse and her transformation into a villain - turned - anti - hero. Nightwing is a skilled fighter and a natural leader. Then, create a situation that forces them to interact, like a mystery in Gotham that they both need to solve. Write their dialogue in a way that shows their personalities. For example, Harley might use her unique sense of humor and Nightwing would be more serious but with a touch of sarcasm.
Another idea is that the father of the baby is a mystery, and Harley goes on a journey to figure out who it is. Along the way, she discovers things about herself and her past relationships. This could involve her visiting old friends or foes and reminiscing about different parts of her life.
Poison Ivy might be a candidate in some creative fanfiction interpretations. Their relationship has always been very close, and if there was some kind of magical or experimental situation, Ivy could potentially be the other parent. This would lead to a very unique family dynamic and story arc.
The Joker is an obvious choice as he has a long - standing relationship with Harley Quinn in the original lore. Their complex and often toxic relationship could add a lot of drama to the pregnancy storyline.
One possible storyline could be Harley Quinn adjusting to the idea of motherhood while still dealing with her chaotic life. Maybe she's trying to figure out how to raise a child in a world full of villains and heroes. Another could be her relationship with the father of the baby, whether it's the Joker or someone else, and how that dynamic changes during the pregnancy.
One great fanfiction could be 'Harley and Nightwing: An Unexpected Bond'. In it, they might start off as enemies due to their different associations in the Gotham underworld. But as the story unfolds, they discover they have more in common than they thought. Maybe Harley starts to question her loyalty to the Joker when Nightwing shows her a different side of heroism. Another could be 'Love in the Shadows of Gotham' where their love has to be kept a secret because of their different standings in the city.