A theme that frequently shows up is the idea of family. Harley Quinn, who has had a rather chaotic family life herself, tries to create a better family for the baby. Also, the theme of growth is there. As she takes care of the baby, she grows as a person, learning things like patience which she didn't have much of before.
Family is also a theme. Some fanfictions might explore what if Harley Quinn had a different family. Maybe she is adopted by a loving family who tries to shield her from the dark side of Gotham. This could lead to a story about her growing up with a new sense of self - worth and different values from the Harley Quinn we know.
Well, the fanfiction could start with Harley Quinn finding out she's pregnant. The Joker is shocked at first but then decides to keep the baby as part of his 'grand plan'. As the baby grows, they have to deal with raising it in their chaotic world. Harley tries to give the baby a normal life as much as possible, while the Joker is constantly getting into trouble that puts the family in danger. It could also explore how the baby affects their relationship, whether it brings them closer or drives them further apart.
One idea could be that the baby has the Joker's chaotic nature but Harley's charm. For example, the baby might play pranks on the henchmen in a really cute yet mischievous way. Another could be that the baby has some special powers, like being able to make people laugh uncontrollably just like the Joker, but also has Harley's acrobatic skills at a very young age. Maybe the baby could be the key to a new, more 'family - oriented' life for the Joker and Harley, leading them to try and reform for the sake of their child.
One popular baby Harley Quinn fanfiction could be 'The Adventures of Baby Harley'. It might show a young and innocent version of Harley Quinn getting into cute, non - violent misadventures in Gotham. It could have her befriending other characters in a more child - like way, like a young Joker who is also just a mischievous kid.
The relationship can be portrayed as a journey. At first, the Joker and Harley might not know how to be parents. They could be clumsy and make mistakes. But as the baby grows, they start to adapt. The Joker might start to soften a bit, realizing that his actions could affect the baby's future. Harley would be the glue that holds the family together. She could be the one who arranges family dinners, even if it's just pizza in their hideout. The baby could also bring out a competitive side in them, like who can teach the baby the coolest trick or skill.
One popular Harley Quinn baby fanfiction could be 'Harley Quinn's New Adventure with Baby'. In this story, Harley might discover a baby in an unexpected situation, like in an abandoned building she was exploring. She then decides to take care of the baby while still being her chaotic self. Another could be 'Harley Quinn and the Mysterious Baby'. It could involve Harley trying to figure out the origin of the baby and facing various challenges along the way.
One idea could be that Harley Quinn and Joker have a baby, and the baby inherits their chaotic and unpredictable nature. Maybe the baby starts causing mayhem in Gotham from a very young age, and the other villains and heroes are all shocked and don't know how to deal with this new situation.