One possible storyline could be Percy having to deal with the constant gloom and the various creatures that lurk in the underworld. He might try to find a way to bring some light or hope to the place. For example, he could use his powers to make small areas more habitable for the lost souls there.
Perhaps Percy discovers a hidden part of the underworld that no one else knows about. This could lead to him finding secrets about the gods or even a way to change the nature of the underworld itself. He may encounter new and strange creatures that are unique to this hidden area and form alliances or face new enemies.
In a fanfiction where Percy lives in the underworld, he could be on a mission to redeem the souls of some of the most heinous criminals. This would be a huge task as the underworld is filled with souls that have committed great evils. He would have to use his wits and powers, and maybe even make some difficult bargains with the rulers of the underworld to achieve this goal.
One possible storyline could be Percy leading a Titan rebellion against the Olympians. Since he has a connection to the Olympian side as well, it would create a very interesting internal conflict for him.
One possible storyline could be Percy forming a band with his demigod friends. They could go on a tour across different mythical realms, performing for gods and monsters alike.
One popular storyline could be Percy's struggle with his new identity as the son of the Underworld. He might face discrimination from other demigods who are afraid or distrustful of his heritage. Another could be his adventures in the Underworld itself, exploring its dark and mysterious regions, perhaps even trying to reform some of the more cruel aspects of the Underworld. And then there are storylines where he has to team up with other unexpected characters, like some of the minor gods or spirits from the Underworld, to save the world from a new threat that only his unique powers and knowledge of the Underworld can combat.
One possible storyline could be that they discover their sibling relationship while on a quest together and have to deal with the shock and how it changes their view of each other.
A possible storyline could be that Percy is on a quest and gets lost in a different dimension which happens to be Gotham. Bruce Wayne, as Batman, finds him and initially suspects him to be a threat. But as they interact, they discover some similarities in their abilities and goals. They then start to dig deeper and find out they are related through a long - lost family connection that dates back to ancient times when the gods had some influence on the early human civilizations.
Perhaps Percy is a single dad because his partner was lost in a battle against the Titans or some other great evil. In the fanfiction, he could be dealing with the grief of that loss while still being a great father. He might take his child on adventures around the places he used to go as a teen demigod, like the Labyrinth, but this time it's to teach his child about their heritage and how to survive in a world full of mythological threats.
One possible storyline could be Percy losing hope after a major betrayal by a close friend in the demigod world. Maybe he feels like he can't trust anyone anymore and starts to isolate himself from his usual group. This could lead to a downward spiral where he even questions his own abilities and purpose.
In some fanfictions, the storyline might involve a mix - up of magic. Percy's powers from the Greek gods could be pitted against the dark magic of Voldemort's heir. For example, Percy's ability to control water might be used to counter the heir's fire - based dark spells. There could also be a storyline where Percy is sent into the wizarding world by the gods to stop the rise of Voldemort's heir because they fear the chaos it could bring to the entire multiverse. And of course, there could be a love story where a character from Percy's world falls in love with someone related to Voldemort's heir, creating a complex web of relationships.
One possible storyline could be the OC being a new demigod who catches the attention of both Percy and Artemis. Maybe the OC has a special power that is crucial in a new threat that's emerging in the world of the gods. For example, the OC could have the power to control time briefly, which becomes important when an ancient enemy tries to rewrite history to gain power.
In the fanfiction, Bella could be involved in a mystery within the orphanage. There might be a secret room or a strange event that keeps occurring, and she takes it upon herself to solve it, which could also lead to her finding out about her real family or some other important aspect of her past.