In the fanfiction, the third child could be the key to solving a mystery that has been plaguing the wizarding world for years. Their birth might be somehow connected to an ancient prophecy. As they grow up, they start having strange dreams or visions that lead them on a quest to uncover the truth, with the help of their family and friends of course.
One possible storyline could be that the third child discovers they have a unique magical ability that neither Harry nor Ginny have. For example, they might be able to communicate with magical creatures in a way no one else can. This could lead to all sorts of adventures as they explore this new power and how it can be used for good in the wizarding world.
The remnants of Voldemort's followers could still be a threat. They might see the third child as a symbol of the new hope in the wizarding world that they want to destroy. Maybe they believe that the child has some sort of power that could ultimately be used to completely wipe out their dark ideology, so they target the child.
One possible reason could be miscommunication. Maybe they had a big fight over how to raise their child and Ginny, feeling unheard, decided to leave. Another reason might be external pressure. For example, if some dark forces were targeting their family again and Ginny thought leaving would keep Harry and the child safer. Or perhaps Ginny had some personal issues she couldn't deal with while being in the family, like feeling she lost her own identity in the relationship.
Well, it could be that Ginny is pregnant in a world where magic exists (assuming this is related to a magical universe like Harry Potter if Ginny is from there). The pregnancy could bring about some sort of magical phenomenon. Maybe the unborn baby has some sort of special power that starts to manifest during the pregnancy. This could lead to Ginny having to protect the baby from those who might want to use the baby's power for their own gain. At the same time, Ginny has to deal with the normal physical and emotional aspects of pregnancy like morning sickness and mood swings.
Perhaps Ginny discovers some hidden secrets or powers within herself as she copes with Harry's death and the pregnancy. Maybe she has to deal with people who question her ability to raise the child without Harry. The story could also focus on how the child grows up with the legacy of Harry, and whether the child inherits any of Harry's special abilities or traits.
It could be someone completely new, an original character created for the fanfiction. Maybe a mysterious wizard who had a brief encounter with Ginny.
One possible storyline is that the Potter Manor is under attack by a new dark force. Harry and Ginny have to defend it using their skills and the help of their friends. Another could be that they are renovating the manor and discover some long - lost family secrets during the process.
In fanfiction, their first child might face some challenges. Since Harry is so well - known, the child could feel a lot of pressure to live up to his father's reputation. The story could revolve around how the child overcomes this pressure, perhaps with the help of Ginny's wise advice and the support of their friends. The child could also have a special bond with Ron and Hermione's children, and they could all get into mischief together at Hogwarts.
One possible reason could be that Harry has been spending too much time on his heroic duties and not enough time with Ginny. Another might be that he made a decision without consulting her, like taking on a particularly dangerous mission. It could also be something as simple as he forgot an important date or anniversary.
One possible storyline could be that Harry's child discovers some long - hidden secrets about Ginny that make them question her character. Maybe the child stumbles upon an old diary or a memory that shows Ginny in a less - than - favorable light. Another could be that Ginny's actions in the past start to have consequences that affect Harry and their child's relationship with her. For example, if Ginny made some enemies long ago and now those enemies target their family.
One possible storyline could be that Harry is wrongly accused of something at Hogwarts, which makes him an outcast. Ginny, being the strong - willed character she is, might be one of the few who still believe in Harry. She could secretly help him clear his name while also developing deeper feelings for him during this difficult time.