One possible storyline could be that Harry is cursed by a dark wizard. The curse has the unexpected side - effect of making him pregnant. He then has to search for a way to break the curse while dealing with the physical and emotional turmoil of pregnancy. Another could be that it's a result of some experimental magic gone wrong. He might have been trying to help someone with a fertility spell and it backfired on him.
Well, in some fanfictions, it could be a consequence of a new type of magic that Harry was unknowingly exposed to. He starts experiencing the symptoms of pregnancy and has to rely on Hermione's research skills to figure out what's going on. Another possible story is that an enemy of Harry's has found a way to use his body as a vessel for something dark. But instead of the expected result, it causes a pregnancy - like state. Harry then has to fight against this internal 'enemy' while also dealing with the external reactions of the wizarding community.
A storyline could involve a magical artifact. Harry discovers an ancient artifact that, when he touches it, causes him to become pregnant. As he tries to figure out what to do, he has to hide it from his friends at first because he's ashamed and confused. Or perhaps it's part of a prophecy that no one understood correctly. The prophecy was thought to be about the downfall of Voldemort, but instead, it's about Harry's pregnancy and the changes it will bring to the wizarding world.
Another storyline could involve the reactions of their friends. Hermione and Ron might be there to support Ginny throughout her pregnancy. There could be a lot of scenes of them sharing their experiences and knowledge about having a baby, adding a touch of friendship and camaraderie to the story.
One possible storyline could be that they are in a committed relationship and the pregnancy is unexpected but they decide to face it together. They might struggle with how it will affect their hero careers and deal with the reactions of their friends and families.
One possible storyline could be that Sirius initially panics when he finds out about the pregnancy. He might struggle with his responsibilities as he has a rather wild and carefree nature in the original story context. He could then gradually grow into the role of a father - to - be, facing challenges from his friends and family who may have different expectations.
One possible storyline could be that they face a lot of family drama. Their families might be shocked and have different reactions, some may be supportive while others are against it. It could also explore how Ben and Gwen deal with the sudden responsibility and how it changes their relationship from just being cousins to co - parents.
In this fanfiction, it could be that Sasuke returns to the village and he and Sakura start to build a life together. Their friends are happy for them, and then Sakura becomes pregnant. The story might explore how the news affects their friends and family, and also how Sasuke and Sakura grow as a couple during this time.
One possible storyline could be the challenges Bella and Edward face as they prepare for a new life together while dealing with the vampire - human relationship issues. Maybe they struggle to keep Bella safe during her pregnancy considering her human frailty.
One possible storyline could be about the challenges she faces during pregnancy, like dealing with morning sickness and hormonal changes. Another could be how her friends and family support her throughout the pregnancy journey. And perhaps there could be a storyline about her preparing for the new arrival, such as setting up the nursery.
Another storyline might involve Harry having to hide his condition from Voldemort and his Death Eaters, as being in a vulnerable pregnant state makes him an even bigger target. He could also form unexpected alliances with female characters who help him through this difficult time.
Perhaps a story where Hanabi has a goal or a dream that she wants to achieve, and Naruto is the one who encourages and supports her throughout the process. In the fanfiction world, this could be symbolically represented as Hanabi being 'pregnant' with this goal. Naruto's presence and influence are so strong that it's like he's the catalyst for this 'pregnancy'. The story could then be about their adventures together as Hanabi works towards achieving her dream, with Naruto by her side as a friend and mentor.
One possible plotline could be how Harry and Ginny deal with the unexpected pregnancy. They might face challenges from their families and friends. Maybe they have to quickly adjust their lives, like Harry taking on more responsibilities to support the family. They could also face some magical complications during the pregnancy that are unique to the wizarding world.