A really interesting plot could be Percy's diary entries during a time when he is separated from his friends and has to rely on his own wits to survive in a strange new place. He could write about the challenges he faces, the new allies he makes, and his longing to be reunited with his friends.
One idea could be that Jackson discovers a hidden magic in his diary that transports him to a different dimension every time he writes in it. Another could be that the diary holds the secrets of his family's past and he has to solve a mystery to unlock them. Maybe the diary has a sentient entity inside that only he can communicate with and it helps him with his daily life problems.
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.
One plot idea could be Percy having to go on a quest specifically guided by Poseidon's hidden messages in the sea currents. He has to decipher these messages to find a lost artifact that can save the underwater kingdom from a new, mysterious threat.
One idea could be Percy using his devilish powers to protect his friends in a unique way. Maybe he has the power to manipulate shadows and uses this to create illusions to mislead enemies.
One idea could be Percy exploring his newfound godling powers and how they differ from his demigod abilities. He might accidentally cause some chaos in the mortal world while trying to master them.
One idea could be Percy helping a Naiad protect her water source from pollution. He uses his demigod powers to fight off the forces causing the pollution.
Maybe Percy gets caught up in a political intrigue within the 'wanderer' world. He has to navigate through different factions, each with their own motives and secrets, while still trying to stay true to his own values.
One idea could be Percy joining a high - stakes tournament. He uses his unique skills and quick thinking to outwit his opponents. Another could be Percy playing in a team where he has to deal with a difficult teammate but manages to turn the situation around and lead the team to victory.