One idea could be that Percy is reborn in a different era, like the Victorian age. He has to deal with new versions of the old monsters while also trying to figure out how his past life ties in. Maybe he discovers a long - lost prophecy about his rebirth that sets him on a new quest.
Another plot could involve Percy being reborn as a half - human, half - something else entirely. For example, half - human, half - nymph. This new form gives him different powers and weaknesses, and he has to adapt to a world that doesn't quite understand him. His friends from his past life might not recognize him at first, and he has to prove who he is.
A really cool plot could be that Percy is reborn, but all the memories of his past are locked away in his subconscious. As he grows up, he starts having strange dreams and visions. Eventually, he meets some new friends who are also connected to the demigod world in some way. Together, they embark on a journey to unlock Percy's past and save the world from a new, unknown threat that is related to his rebirth.
One idea could be being reborn as a new camper at Camp Half - Blood. You could discover new powers related to water, like being able to create ice sculptures with just a thought, which is a unique take on Percy's water - based powers. Another plot could involve being reborn as a child of a minor god, like Hecate. You'd then have to struggle to prove your worth among the children of the major gods. And perhaps a plot where you're reborn with memories of the future, and you have to use that knowledge to prevent a great disaster at Camp Half - Blood.
Well, one interesting idea could be Percy having new powers or abilities. Maybe he's reborn with the power to control not just water, but also other elements like fire or air. Another could be his relationships being different. For example, his friendship with Grover might start off on a more competitive note instead of being friendly from the start. And also, the whole concept of him being reborn might mean he has a different view of the gods, perhaps more skeptical or more understanding depending on his new experiences.
Perhaps being reborn as a child of Poseidon but with a twist. Instead of having the typical water - based powers, you have the power to communicate with sea creatures in a way that no one else can, like understanding their emotions and memories. Or being reborn as a demigod who has lost their memory but slowly regains it as they go on quests and face challenges, and each recovered memory reveals a crucial part of their past and a new power or ability.
A possible plot is that the reborn female OC starts to form a different kind of relationship with the existing characters. Since she's female, her friendship with Annabeth could be more of a sisterly bond where they support each other in different ways compared to Percy and Annabeth's relationship. And she might have a romantic sub - plot with a new character, someone who was not really in the spotlight in the original series. This new relationship could be shaped by her unique identity as a reborn character.
One plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.
One idea could be Percy and his friends getting trapped in a virtual game world that's a twisted version of their own. They have to use their demigod skills to defeat the digital monsters and find a way out.
Maybe Percy is sent on a quest to Atlantis because there are rumors of a traitor among the Atlanteans who plans to side with the Titans. Percy has to navigate the complex political situation in Atlantis and figure out who the traitor is before it's too late. He also has to deal with the Atlantean magic which is different from what he's used to.
One idea could be that Percy discovers a hidden realm within the world of the Wanderer. This realm has its own set of rules and challenges that he has to navigate. Maybe he meets new demigods there who have been lost for centuries.
One idea could be Percy having to make a deal with a minor underworld god to save Annabeth from a new and strange curse that's slowly draining her life force. He has to complete a series of tasks in the underworld that test his loyalty and courage more than ever before.
Well, perhaps the shield was once owned by a powerful demigod from the past. In the fanfiction, Percy starts having strange dreams about this demigod and the shield's true origin. As he investigates, he realizes that the shield holds the key to defeating a new, emerging evil that threatens not just Camp Half - Blood, but the entire world of gods and mortals. So, he has to figure out how to unlock the shield's full potential.