One interesting plotline could be Percy using his powers to save his school from a mythical threat that somehow infiltrates the campus. For example, a group of minor demons might be causing chaos in the school, and Percy has to use his water - controlling abilities to stop them without revealing his identity to most of the students.
Another plotline might involve Percy's friends from Camp Half - Blood visiting the school. They could be disguised as new students, and there could be a whole sub - plot about how they try to fit in while also being on the lookout for any monsters that might be targeting Percy at school. This would create a lot of comical and action - filled situations as they navigate the normal school life while being demigods.
Some fanfictions might focus on different pairings. Like Percy with someone other than Annabeth in an alternate universe kind of scenario. It could also delve into the powers of the characters more, perhaps showing Percy developing a new aspect of his water - related abilities that was only hinted at in the official stories.
There might be fanfictions that focus on the relationships between different gods. Like a story about the friendship (or rivalry) between Poseidon and Hades. It could explore their past, their different views on ruling their realms, and how Percy and other demigods get caught in the middle of their power struggles.
In some fanfictions, there are often crossovers. For example, a demigod from the Percy Jackson world meeting a character from another popular fantasy series. This can lead to really cool and unexpected adventures. They might have to combine their different skills and knowledge to overcome a common enemy. The contrast between the two different fictional universes can create a lot of interesting plot twists.
A possible plotline could be that the hermit is actually a forgotten hero from the past. He has been in hiding all this time but now his past is catching up with him. As he tries to keep his identity a secret, events unfold that force him to face his former life and the consequences of his actions long ago.
One interesting plotline could be the Seven facing a new, more powerful enemy that is a combination of different mythological creatures. This would test their teamwork and individual powers in new ways.
One interesting plotline could be Percy having to face a new and powerful ancient evil that the gods are also afraid of. This would put him in a position where he has to use his wits and powers in ways he hasn't before.
Maybe in the fanfiction, Percy's player side shows up when he has to navigate the complex social hierarchies of the demigod world. He could be seen charming different groups of demigods, from the children of Athena with his quick - witted remarks to the children of Aphrodite with his newfound 'player' charisma. This could lead to all sorts of adventures and conflicts as he tries to maintain his relationships while also keeping up his player image.
One interesting plotline could be the siblings going on a secret quest together without the knowledge of Chiron or the other campers. They might face a new and unknown monster that tests their combined skills.
In some fanfictions, a centurion discovers a long - lost prophecy that involves Percy. This could lead to a race against time as they try to figure out the meaning of the prophecy and prevent a great disaster. Maybe it involves a new Titan rising and they need Percy's unique connection to the sea gods to stop it.
Another plotline could be Percy trying to find a way to clear his name and escape the prison. Along the way, he might make some unexpected allies among the inmates or even some prison staff who are sympathetic to his cause. This could lead to an epic adventure within the prison walls as he tries to gather evidence and outwit his captors.