Another plot idea is Percy exploring a forgotten part of the Roman camp. There he discovers an ancient prophecy that seems to be related to him but has been lost in time. He then has to figure out how this prophecy ties into his role as the Son of Neptune and what it means for his future.
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.
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.
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.
One idea could be Percy having to face a new and powerful sea monster that is immune to his normal powers. He has to rely on his wits and the help of some unexpected allies from the Roman side. Another could be Percy getting lost in an underwater maze that was created by an ancient sea god as a test for him. Maybe he discovers a long - lost relative who is also a demigod and they team up to solve a mystery that threatens both the Greek and Roman camps.
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.
One idea could be Percy having to use his charm powers (from being Aphrodite's son) to charm a powerful, ancient monster into sparing a group of demigods. He might struggle with the morality of using such powers in this way.
One idea could be Percy using Wolverine's healing factor during a particularly tough battle in the world of Greek mythology. Maybe he gets injured by a powerful monster and the unexpected emergence of the healing ability saves him and surprises his friends.
One idea could be Percy having to go on a quest with the children of Dionysus. They could face a new type of monster that is immune to normal weapons but can be defeated by the power of wine, which the children of Dionysus can control. Another could be Percy getting into a friendly rivalry with a son of Dionysus over who can throw the best party at Camp Half - Blood. Maybe they compete to see who can get more demigods to attend their party, and in the process, they learn more about each other's powers.
Perhaps Percy finds out that being the son of Selene gives him a connection to the dreams of mortals. He starts to have strange visions through their dreams and realizes that there is a hidden threat that is using the dreams to manipulate people. He has to figure out how to stop this threat without causing harm to the mortals. He might team up with some other demigods who have powers related to the mind or dreams to solve this mystery.
One idea could be Percy having to face challenges specific to being the 'son of Lycaon'. For example, he might struggle with the werewolf - like instincts that come with that heritage. He could have to prove himself to the other demigods who are skeptical of his newfound origin.
One idea could be Percy having to face a new and powerful enemy that is specifically targeting the Roman demigods. He could team up with some of his old friends from the Greek side and form an alliance with the Romans to defeat this threat.