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.
There might be some out there. You could start by looking on fanfic platforms like Archive of Our Own. Sometimes they have unique takes on the relationship between Batman and Harley Quinn, and a pregnancy storyline could add an interesting twist to their dynamic.
A prevalent theme is often about relationships. It could be her relationship with the father of the child, if it's the Joker, how that dynamic changes. Or it could be about her new relationships she forms with other characters who support her during the pregnancy, like other female characters from the DC universe.
One common theme is Harley's struggle with her new identity as an expectant mother. Given her chaotic and often unpredictable nature, the fanfics might explore how she copes with the idea of bringing a new life into the world.
One possible plot could be that Harley Quinn discovers she's pregnant and tries to hide it from the Joker, fearing his reaction. Meanwhile, Batman somehow gets wind of it and feels a sense of responsibility to protect her, not just for the baby's sake but also because he realizes Harley is in a very vulnerable state.
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.
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.