One possibility could be that Percy has been leading a double life. Maybe he had a relationship in the past that he kept hidden, and now the existence of the child is coming to light. The story could explore how he copes with this new reality and how it affects his relationships with his friends and family.
Well, perhaps the secret child has some special powers or abilities. In the fanfiction, it could be that Percy discovers the child is in danger because of these powers, and he has to protect the child while also dealing with the shock of having a secret offspring. This could lead to a series of adventures where Percy learns more about himself as a father.
It could be an old acquaintance that Percy had a brief relationship with. Maybe someone he met during one of his adventures and then lost touch with, but she later discovers she's pregnant and decides to keep the child a secret from Percy for various reasons.
One possible plot could be that Naruto discovers he has a child during a mission. Maybe he saved a village years ago and unknowingly had a relationship with a local woman. The child could be in danger from some villains who want to use the child's connection to Naruto for their own evil plans.
Maybe the plot would revolve around the relationship between Percy and the goddess. It could show how they met, fell in love, and decided to have a child. Then it might explore how the other gods and goddesses react to this union and the birth of the child.
One possible plot could be that Fem Naruto has been in hiding with her child to protect it from old enemies. Maybe she was in a relationship during a time of peace but things turned bad. Now she's constantly on the move, training her child in secret ninja skills.
The plot could involve the child suddenly showing up at Percy's doorstep one day, and Percy having to deal with the shock and figure out how to fit this new part of his life into his already busy schedule at the Ministry. He might struggle with how to tell his family, especially after all the past tensions.
Perhaps the story could focus on Percy's child trying to live a normal life in the mortal world but constantly being drawn back to the world of the demigods. They could struggle with fitting in at school while also having to deal with strange occurrences that only they seem to notice, like seeing mythical creatures in the corner of their eye. This internal conflict between two worlds could drive the plot forward.
Perhaps the plot could involve Percy's daughter discovering her true identity on her own. She might start to notice strange things about herself or her father's behavior. For example, she could find some old letters or artifacts that hint at her origin. And as she digs deeper, she gets closer to the truth, which also brings her closer to the dangerous world her father has been shielding her from.
It could be a mark that represents his family or a long - lost heritage. Perhaps it's a crest that has been passed down through generations in his family but he has to keep it secret for some reason, like it might draw unwanted attention from certain enemies or factions in the story.
Well, one possibility could be that Percy has been on some sort of adventure and during that time, he discovers a child in need. He decides to take the child under his wing and brings the child back to his world. Then, he has to deal with the reactions of his friends and family, who are both surprised and a bit worried about this new addition.
Maybe it could be a new character. A mysterious woman who Percy meets during one of his adventures. This would add an element of mystery to the story.