One plot idea could be Percy discovering he has the ability to bloodbend and struggling to control it. He might be afraid of the power and how it goes against his usual water - based abilities. Another could be Percy having to face an enemy who uses bloodbending in a malicious way, and he has to learn it quickly to defend himself and his friends.
Well, a really interesting plot could involve Percy being the only demigod with the bloodbending ability. This sets him apart from his peers and makes him a target. The gods might be wary of him having such a powerful and somewhat 'dark' ability. Or perhaps Percy has to use bloodbending to save someone he loves from a curse that can only be broken through this power. He would have to grapple with the moral implications of using it in the process. There could also be a plot where Percy is tricked into using bloodbending against his friends, and then has to find a way to reverse the damage.
A common theme is power and its consequences. Percy having the bloodbending ability makes him very powerful, and often fanfictions explore how he deals with the responsibility that comes with it. Another theme is identity. His new power can change how he sees himself and how others perceive him.
One idea could be that Percy is framed for stealing a powerful magical artifact. His friends initially doubt him, but as he tries to prove his innocence, he uncovers a conspiracy within the magical community. For example, a secret group wants to use the situation to gain more power and is setting him up as a scapegoat.
One idea could be that Percy has to seek Athena's wisdom to solve a complex mystery in the mortal world that is somehow related to the gods' ancient secrets. Maybe it involves a powerful artifact that was lost long ago and only Athena's knowledge can lead Percy to it.
One idea could be Percy joining the hunt temporarily. Maybe there's a prophecy that requires his unique skills and the hunt needs his help to complete a mission. For example, a powerful monster is on the loose that only Percy can track down because of his connection to the sea. Another could be a love triangle between Percy, a huntress, and someone from Camp Half - Blood.
One idea could be Percy joining a high - stakes tournament. He uses his unique skills and quick thinking to outwit his opponents. Another could be Percy playing in a team where he has to deal with a difficult teammate but manages to turn the situation around and lead the team to victory.
One idea could be a time - travel plot where Percy goes back in time and meets a younger, different Medusa before she was turned into a monster. Another could be a story where Medusa somehow gets a chance at redemption and Percy has to decide whether to trust her or not. Maybe a plot where they have to team up to defeat a greater evil that threatens both of them.
One idea could be Percy exploring his newfound godling powers and how they differ from his demigod abilities. He might accidentally cause some chaos in the mortal world while trying to master them.
One idea could be Percy helping a Naiad protect her water source from pollution. He uses his demigod powers to fight off the forces causing the pollution.
One idea could be Percy using his devilish powers to protect his friends in a unique way. Maybe he has the power to manipulate shadows and uses this to create illusions to mislead enemies.
A really interesting plot might be a love triangle situation. Maybe Percy is in love with Katniss, but Katniss is involved with someone from her own world, like Gale or Peeta, and Percy has to compete in some way. This could add a lot of drama and tension to the story. Also, they could be in a post - apocalyptic world together, where they have to rely on each other's survival skills more than ever.