Another idea is that Sirius is in a complex situation where he is involved in some dangerous or important mission at the same time as the pregnancy. He has to balance his duties towards the girl and the unborn child with his obligations in this other aspect of his life. He might have to make difficult choices between protecting his loved ones and fulfilling his other responsibilities.
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 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.
It could be a new character created specifically for the fanfiction. Maybe a witch who Sirius meets during one of his adventures away from his usual group of friends.
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 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 storyline could be that Ash and Serena have been traveling together for a long time and their relationship deepens. They might face some initial shock and then start to plan for their future together. They could have to deal with reactions from their friends and family, like Pikachu being both excited and a bit confused.