It could be that Percy is sent to prison because he's seen as a threat to the established order of the demigod world. Some of the more conservative or power - hungry leaders might fear his growing power and influence. So, they fabricate a reason to imprison him. In prison, he uses his skills and wits to form alliances with other inmates who also feel wronged. Together, they plan to break out and expose the corruption of those who put him there, all while Percy continues to develop his powers in secret.
Well, one possibility could be that Percy is framed for a crime he didn't commit. Maybe some powerful enemy uses magic or trickery to make it seem like he's violated a major demigod law. So, he's sent to a special prison for demigods. Inside, he has to deal with all sorts of other dangerous inmates, some of whom might be old enemies or just generally bad - tempered demigods. He'd probably try to find a way to prove his innocence and escape while also having to survive the various challenges within the prison.
There could be an ancient and evil force that has infiltrated the demigod world. This force uses Percy as a scapegoat and arranges for him to be sent to prison. It wants to cause chaos and destruction within the demigod community. It manipulates the situation from the shadows, and Percy has to figure out its identity and motives while in prison in order to stop it.
The plot could center around Percy's struggle to fit in at college while still being a hero. He may find that the other students don't understand his need to disappear suddenly when there's a monster attack. He could also get involved in some sort of college - wide mystery that has roots in the ancient world. Like, strange symbols start appearing all over campus that only he can decipher because of his knowledge of Greek and Roman mythology. And he has to team up with some unlikely allies, like a nerdy professor who turns out to have a hidden knowledge of the demigod world, to solve the mystery and save the college from an impending doom.
In a 'naruto goes to prison' fanfiction, it could be that Naruto is tricked into taking the blame for someone else. Once in prison, he meets old foes who are also incarcerated. Instead of fighting them, he tries to understand their motives and in the process, he uncovers secrets about their pasts that were related to some greater threat to the ninja world. This knowledge then becomes crucial for his future actions outside the prison.
Well, one possibility could be that Percy discovers a new aspect of the mythological world. Maybe in this fanfic, his death is a sacrifice to save his friends. Once in Valhalla, he could meet new heroes from different mythologies and form alliances or rivalries. He might also try to find a way back to life or to his friends in the world he left behind.
He could meet some Aboriginal heroes or spirits. They might have their own powers and knowledge that Percy can learn from.
In fanfiction, if Percy Jackson goes to highschool, he might face challenges like hiding his demigod powers from his classmates. He could also encounter new mythical creatures that are drawn to the school environment.
In a fanfiction where Percy Jackson goes to Azeroth, he could form alliances. Since he's used to being in a team in his own world, he'd likely team up with some of the heroes in Azeroth. For example, he could join forces with Thrall and help him in his quest for the Horde. His sword - fighting skills from Camp Half - Blood would come in handy in battles against the Burning Legion or other enemies.
If Percy Jackson goes to UA, there would be a huge culture clash at first. The UA students are used to a very structured hero training system, while Percy is more of a 'go with the flow' kind of demigod. But over time, he could teach them about improvisation in battle and they could teach him about teamwork in a more organized way. He might also introduce the concept of Greek mythology to the UA world, which could open up new ways of thinking about powers and abilities.
Well, he might get into all sorts of adventures. He could meet the dragons in Alagaesia and try to use his water powers to his advantage against them. Maybe he'd team up with the local heroes there and learn new combat styles.
Perhaps Percy would see a future where the monsters have evolved in strange ways. They could be more intelligent and organized, posing an even greater threat to the demigods. He might also find that some of his old friends have made great sacrifices and that he has a new role to play in this future world, like being a mentor to the new generation of demigods.