It could be someone completely new, an original character created for the fanfiction. Maybe a mysterious wizard who had a brief encounter with Ginny.
It could be someone Ginny had a past relationship with before Harry. Maybe an old schoolmate or a character from a side plot in the fanfiction.
One plot twist could be that Harry is actually protecting Ginny and the real father. He might know some dark secret about the father that would put them in danger if it were known, so he pretends to be in the dark about the situation.
It could be a case of misinformation. Someone in the story might have spread false rumors about Harry being the father for their own motives. Perhaps they wanted to cause chaos or gain something from the situation. So, the whole idea of Harry being the father is just a false story that gets spread around the wizarding community.
It could also lead to a mystery aspect in the fanfiction. Harry, being the hero he is, might start to investigate who the real father is. This could uncover a lot of secrets in the wizarding world, secrets that might be related to old families, forgotten spells, or hidden alliances. The whole story could then spiral into a complex web of mystery and drama as Harry delves deeper into the truth.
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.
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.
Maybe Ron Weasley. Imagine a fanfiction where Ron, after getting married, has triplets. Given his humorous and sometimes clumsy nature, it would be really fun to see how he and his wife deal with three little ones. He could pass on his love for food and his loyalty to his children in a really endearing way.
A possible plotline might involve a magical experiment. Harry, being the curious wizard he is, participates in an experiment that has unforeseen consequences on the fabric of time and family relations. As a result, he becomes Hermione's father. The story could then follow how Hermione discovers this truth and how it affects her relationship with Harry. It could also explore how their friends like Ron react to this strange new family dynamic in the wizarding community.
One plotline could be that the de - aged Voldemort, now in a more innocent state, tries to make amends for his past actions towards Harry. He might use his knowledge of magic to protect Harry from other threats in the wizarding world. Another could be that Harry initially resists the idea of Voldemort as a father - figure but slowly comes to understand that this new Voldemort is different. And then there's the plotline where the wizarding world is in chaos because of this strange relationship, with some people believing it's a dark plot while others see it as a chance for redemption.
One possible storyline could be that Snape secretly tries to guide Harry without revealing his true identity. He might give Harry special potions or spells to help him, all while acting as the strict professor. Another could be that as Harry discovers the truth, he has to deal with a whole new set of emotions towards Snape.