A common theme is friendship. Their unique bond, whether it's a platonic or a more romantic - leaning one, is often explored. It shows how they support each other through Gotham's chaos.
One great Harley Quinn and Poison Ivy fanfiction is 'Green Love'. It beautifully portrays their relationship in a more innocent yet deep way. It focuses on their shared adventures and how they support each other through different challenges.
There could be a plot where Poison Ivy discovers a new type of plant that has the power to make people tell the truth. Harley gets curious and tries it on some of the bad guys in Gotham. Meanwhile, Ivy is worried that this new plant could attract unwanted attention from the government or other powerful forces. They have to decide whether to keep it a secret or use it for their own purposes.
Perhaps the story could focus on how Harley Quinn and Poison Ivy adjust to their new family dynamic. Harley might be the more impulsive and fun - loving parent while Poison Ivy is more protective and nurturing. The baby could also bring out a softer side in their villainous personas, leading them to consider a more peaceful existence.
I highly recommend 'Partners in Crime'. In this fanfic, Harley and Ivy team up to take on some new villains in Gotham. It's full of action and at the same time, shows their relationship evolving. Another one is 'Under the Stars'. It has a more romantic tone as they go on a camping trip together, away from the city lights, and discover new aspects of their relationship while enjoying the beauty of nature.
Their relationship could be portrayed as a kind of mentor - mentee thing at first. Poison Ivy teaching Harley about the power of nature and how to use it. Then it slowly turns into a deeper friendship and more of an equal partnership.
Character development is key. Harley's journey from being Joker's sidekick to finding her own identity with Ivy by her side. And Ivy's growth as a character, her connection to nature and how that affects her relationship with Harley. Also, a unique plot. Maybe something like they go on a quest to save a part of Gotham's nature that's being destroyed, which ties in with Ivy's character. It should also have good dialogue that reflects their personalities, like Harley's crazy but endearing one - liners and Ivy's more serious, nature - centric statements.
The baby might have Ivy's green - tinted skin, a sign of her connection to plants. And perhaps Harley's bright blue eyes and a wild, unruly shock of hair like Harley's pigtails.