Some storylines involve Percy using his underworldly powers in creative ways. For example, he could have the ability to communicate with the dead in a more profound way than other demigods. This might lead to him solving ancient mysteries that are tied to the fates of both the living and the dead. Also, there could be a storyline where he is in a love triangle with a surface - world demigod and an underworld spirit, which would add a lot of drama and tension to the story. He would have to navigate his feelings while also dealing with the political and power - based rivalries between the two worlds.
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.
A common storyline could be Percy trying to find his place in the demigod world. Since he is the son of the Underworld, he doesn't fit in easily with the children of the more 'popular' gods like Zeus or Poseidon. He might go on a quest to prove himself worthy, not just to the other demigods, but also to his father, Hades. This could involve facing off against powerful monsters that are specifically associated with the Underworld, like the Cerberus or the Furies. Along the way, he would discover new powers and make allies and enemies within the underworld community.
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.
One popular storyline is Percy falling in love with Aphrodite. In these fanfictions, Aphrodite might see something special in Percy beyond his heroic deeds. Maybe she uses her powers of love to draw him closer, but Percy is conflicted because he knows she's a goddess and he's a demigod. It creates a lot of drama as he tries to figure out his feelings and deal with the power dynamics.
Well, a possible storyline is Percy dealing with the expectations of the camps of Apollo and Athena. Since he's the son of both in this fanfiction, he might be pressured to be the best in both areas like archery and battle strategy. He could also go on a quest where he has to use his Apollo - given musical talent to charm a dangerous monster and then use Athena - taught wisdom to outwit another foe.
One interesting storyline could be the struggle of a Son of Artemis in a world dominated by male demigods. He might face discrimination or be underestimated at first. For example, in a quest, others might doubt his abilities because he's associated with Artemis, a goddess who has a mainly female following. Another could be his relationship with the wild animals that Artemis is associated with. He might have an extraordinary bond with them, which helps him in various challenges, like using a pack of wolves to track down a powerful enemy.
If Percy was the son of Aphrodite, it would make the story much more about relationships and less about the traditional battles. His storylines could involve using his charm to defuse situations rather than his sword. He might be more focused on bringing people together or causing romantic chaos. Also, his connection to the other gods would change. Gods who have a different view of Aphrodite might treat Percy differently. He could be a target for jealousy from some gods' children who feel he has an unfair advantage in the love department.
One popular storyline could be Percy getting a magical haircut. Maybe his hair is cut by a magical creature and it gives him new powers. For example, his hair might be cut by a water nymph and then he can control water in a new way. Another could be Percy getting a haircut as part of a disguise. He might be trying to infiltrate an enemy's camp and a new haircut is part of his transformation. A third storyline could be that Percy's haircut is a result of a bet or a challenge. He loses a bet to one of his friends and has to get a really wild haircut.
One popular storyline could be Percy and Annabeth going on new adventures together, exploring different parts of the mythological world that weren't covered in the books. For example, they could travel to a long - forgotten island full of ancient and powerful magic.
A character like Circe could be really interesting to pair with Percy. Circe has her own magical powers and a connection to the more mysterious parts of the mythological world. Percy could learn a lot from her about magic and how to use his powers in different ways. Also, the dynamic between them would be full of tension and intrigue, as Circe might be tempted to use Percy for her own ends. Another great pairing would be Reyna. Reyna is a strong and independent leader, and when paired with Percy, they could lead different groups of demigods in a joint mission. Their leadership styles would clash and mesh in interesting ways, and they could develop a deep respect for each other as they face challenges related to Percy's underworld heritage.
One interesting storyline could be Percy as a baby showing early signs of his demigod powers. Maybe he can control water in his crib, like making the bath water swirl around him in a mini - whirlpool. Another could be the reactions of the gods when they discover this new baby demigod. They might be worried about his potential or already planning how to use him in their future schemes.
There could also be a fanfiction where Percy has to face his inner demons in a more psychological way. Maybe he's haunted by the memories of all the battles he's fought and the friends he's lost. His friends then have to help him through this tough time, and in the process, they learn more about each other's deeper fears and insecurities.