Maybe the fanfiction he reads in this scenario has an alternate ending to 'The Son of Neptune'. Percy could then be inspired to try and make that alternate ending a reality in his own world, leading to a whole new set of adventures and interactions with the other characters.
It could also be that the fanfiction is written from the perspective of a character in 'The Son of Neptune' that Percy hasn't fully understood before. As he reads it, he starts to see things from that character's point of view, which changes his relationships and actions in his own story.
One possible plot idea could be Percy reading it and getting new insights into the story, which then helps him in his own adventures. For example, he might find a hidden clue in the fanfiction that relates to his real - life challenges.
Well, a great plot for Percy son of Neptune fanfiction could involve Percy getting lost in a time - warp within the ocean. He could end up in an era where Neptune was just starting to gain his powers. This would force Percy to interact with a younger version of his father and also deal with the different rules and threats that existed back then. Additionally, Percy could find a way to bring some knowledge from that time to his present to help in a current conflict. Also, Percy might have to team up with some primitive sea creatures who have their own unique abilities and motives.
Well, he might gain new perspectives on the events and characters in 'The Son of Neptune'. Maybe he'll find alternative versions of his own actions or the actions of his friends interesting. It could also make him reflect on how others view the story.
A really interesting plot would be if Percy was transported to a parallel universe where the roles of the Greek and Roman gods were reversed. He would have to adapt quickly to this new world, make new friends, and fight against the enemies who now have different motives and powers. It would be a great opportunity for character development and exploring different mythologies.
One idea could be Percy forming an unexpected alliance with a minor Roman god. In the fanfiction, this god could have a unique power that helps Percy in his quests. For example, a god of illusions who can create decoys to mislead Percy's enemies.
One plot idea could be Percy discovers some hidden prophecy while reading 'Son of Neptune' that affects his future adventures. Another could be that he reads it and starts to question his own role in the demigod world more deeply.
There could be some on Archive of Our Own as well. It has a vast collection of fanfictions. You need to be a bit more specific in your search though. Maybe add some tags like the genre you prefer, whether it's action - packed or more of a character - study type where Percy reads 'The Son of Neptune'.
Perhaps Percy would feel a sense of pride. Knowing that his story has inspired others to create their own versions shows that his adventures have had an impact. He might also be a bit surprised at some of the wild ideas that fans come up with, but overall, it could be a positive experience for him.
Another plot could involve Percy's journey to discover more about his Roman heritage. He could be sent on a quest by Neptune himself to find an ancient artifact that is key to the future of the Roman demigods. Along the way, he has to deal with the politics and rivalries within the Roman camp. Some might see him as a threat to their power, while others see him as their only hope. He also might have visions of his past lives as a Roman hero which could guide him on his quest.
You could write about the Son of Neptune getting lost in a mysterious realm. In this realm, the rules of his world don't apply. He has to learn new ways to survive and fight. Along the way, he meets a group of misfits who he has to team up with. Together, they try to find a way back home while also uncovering secrets about this strange place that could have implications for his own world.
There's a fanfiction where the relationship between Percy and Hazel is explored in a new light. It shows them training together and sharing their experiences from their different pasts. This one is really good because it adds more emotional depth to their characters.