Another plot could be that Percy has a secret identity as a superhero - like figure. He uses his powers, which he has hidden from everyone, to protect the city at night while maintaining his normal life during the day.
Perhaps Percy's secret life is that he is a time - traveler. He uses his ability to go back in time to fix mistakes or prevent disasters, but he can't let anyone know as it could disrupt the timeline.
One idea could be that Percy is secretly working on a project to reform the Ministry of Magic from within. He might be gathering evidence of corruption in the ministry without anyone knowing. This could add a whole new layer of mystery to his character.
A possible plot for a 'Percy Jackson Secret Power' fanfiction could involve Percy having a secret power of teleportation. But this power is not fully under his control at first. He might accidentally teleport to strange and dangerous places. One time he could end up in the middle of a Titan's lair. As he tries to master this power, he discovers that it has a connection to an ancient prophecy. His friends, like Annabeth, help him figure out how to control it and use it for the greater good.
One idea could be that Percy's child discovers they have special powers like Percy. For example, they can control water in unique ways. Another could be the child getting into trouble at a new school for demigods and Percy having to step in to help.
The son could be cursed by a powerful being. Percy then has to find a way to break the curse. This could involve seeking help from old friends and new allies. Or perhaps the son has a premonition about a great danger coming to the world of demigods, and he and Percy have to convince the other demigods to take action before it's too late.
Another possibility is that his secret wife was an ordinary human at first. They fell in love in Percy's early days, before he became well - known. She's kept secret because Percy wants to keep her out of the dangerous world he often finds himself in. For example, she could work in a library, and they have a very normal - seeming relationship in private.
One idea could be that Percy discovers his ice powers during a dangerous mission. Maybe he's in a really hot and hostile environment, like a volcanic area, and suddenly his ice powers emerge to cool things down and save his friends. Another could be that he has to train with an ancient ice spirit to control his new powers. The spirit could be really strict and Percy has to learn discipline along with mastering his ice abilities.
Well, perhaps Percy could be reborn into a different era in the 'Percy Jackson New Life' fanfiction. Like, he's born in ancient Rome during the height of the Roman Empire. He has to navigate this new world with his old memories intact, and deal with the differences between the Greek and Roman pantheons. He might also meet versions of his old friends in their Roman forms and form new alliances and rivalries.
One idea could be that Percy is framed for stealing a powerful magical artifact. His friends initially doubt him, but as he tries to prove his innocence, he uncovers a conspiracy within the magical community. For example, a secret group wants to use the situation to gain more power and is setting him up as a scapegoat.
One idea could be Percy getting involved in the Kurukshetra war. He might use his water - controlling abilities to change the course of battles near rivers or lakes. For example, he could create huge tidal waves to disrupt enemy formations.
One idea could be a time - travel plot where Percy goes back in time and meets a younger, different Medusa before she was turned into a monster. Another could be a story where Medusa somehow gets a chance at redemption and Percy has to decide whether to trust her or not. Maybe a plot where they have to team up to defeat a greater evil that threatens both of them.