One possible interesting plotline could be Percy's struggle with his newfound power as the demon king. He might have to face challenges from other powerful beings who don't want him in that position.
Another plotline could involve Percy's relationships. As the demon king, his friends from his previous life might be scared of him or try to change him back. Meanwhile, he could form new alliances within the demon realm. These relationships, old and new, would be complex and full of drama as he navigates his role as the demon king.
Percy is the central figure. In addition to him, there could be antagonists. For example, other demon lords who see Percy as a threat to their power. And there might be characters who act as mentors or guides for Percy, even if they are demons themselves. They could help him understand the ways of the demon world. Also, there could be characters from Percy's human world who end up in the demon realm and Percy has to decide how to deal with them.
One idea could be Percy having to form an uneasy alliance with the Demon King to stop a greater threat that endangers both the demigod and demon worlds. Maybe a powerful ancient evil is rising and they have no choice but to work together, despite their initial distrust.
Maybe Percy discovers a long - lost power within himself in Tartarus. This power could be related to his connection with the Underworld. He could use it to communicate with the souls trapped there, and gain knowledge that helps him rule Tartarus. He could also start training with some of the toughest monsters to become even stronger and assert his rule.
One possible plotline could be Percy's struggle with his new godly powers and how it affects his relationships. For example, his friends might be in awe or a bit intimidated by his new status.
One possible plotline could be Percy having to adjust to the unique rules and traditions of the hunters. For example, dealing with their vow of celibacy in a humorous or challenging way. Another could be Percy going on a special hunt with the hunters that involves a mythical creature not commonly associated with them.
One possible plotline could be their adventures in a new, undiscovered part of the mythological world. Maybe they stumble upon a hidden realm full of ancient and powerful creatures that they have to either befriend or fight.
Well, perhaps in this 'percy is an orphan fanfiction', there's a plotline where Percy is constantly bullied because he's an orphan. But then he discovers a hidden power within himself that not only helps him stand up to the bullies but also starts to unravel a mystery related to his family. Maybe he finds out that his parents were part of a secret organization and their disappearance was not accidental.
One interesting plotline could be them teaming up to rescue a captured demigod. They could use their unique abilities, Percy's water - related powers and Bianca's skills with a certain weapon or magic. Another plotline could be them exploring an unknown part of the mythological world together, facing strange creatures and ancient puzzles.
Perhaps a plotline where Percy has a different personality. Instead of being the brave and somewhat impulsive hero, he could be more reserved and calculating. This change in character would greatly affect his relationships with other characters and the way he approaches challenges in the story.
One interesting plotline could be Percy having to protect a high - profile celebrity from a mysterious and dangerous stalker. He uses his unique skills and knowledge to outwit the stalker at every turn.