Perhaps the sister gets kidnapped by an enemy of Percy's. Percy then has to embark on a dangerous quest to rescue her. Along the way, he discovers more about his sister's true nature and their family's past that was previously unknown to him. This could involve meeting old friends and making new allies.
Well, one possibility could be that the sister discovers she has some unique powers that are different from Percy's. For example, she might be able to communicate with mythical creatures in a way that no one else can. This could lead to her getting involved in a quest to save a particular part of the mythical world that is in danger.
One possibility is that the sister has some unique powers that the Olympians fear. So they abandon her. In the fanfiction, she could be on a journey to discover her true self and her powers, while Percy unknowingly gets involved in her quest. Maybe she's being hunted by some mythical creatures because of her heritage, and Percy has to save her in the end.
One possible plot could be Percy feeling a deep sense of betrayal and loneliness. He might struggle with his self - worth as he wonders why his sister left him. This could lead him on a journey to find answers about his family and his own identity, perhaps even uncovering secrets about his demigod heritage that his sister knew and he didn't.
The sister could be under the control of a powerful god or goddess who has a grudge against Percy. She might be forced to betray him. For instance, she could lead Percy into a trap set by the god. She could lie to him about a mission, saying it's a rescue mission but in fact it's a trap where the god is waiting to take revenge on Percy for some past deeds. This would be a really heart - breaking betrayal for Percy as he trusts his sister.
One possibility could be that the sister has a special power like Percy. Maybe she can control water in a different way, like being able to turn it into ice instantly. She could join Percy on his quests and use her power to help them out of tough situations.
Well, one possible storyline could be that Percy and his twin witch sister are separated at birth. Percy grows up in the normal world, while his sister is in the magical witch world. Then, through some strange turn of events, they meet. Maybe there's a prophecy about them that brings them together to fight a great evil that threatens both the magical and non - magical worlds.
It could be an old enemy of Percy's father, Poseidon. Maybe this antagonist blames Percy and his sister for something Poseidon did in the past and wants revenge on the family. So they target Percy's sister as a way to get to Poseidon through his children.
Maybe the plot would revolve around the relationship between Percy and the goddess. It could show how they met, fell in love, and decided to have a child. Then it might explore how the other gods and goddesses react to this union and the birth of the child.
One possibility could be that Percy has been leading a double life. Maybe he had a relationship in the past that he kept hidden, and now the existence of the child is coming to light. The story could explore how he copes with this new reality and how it affects his relationships with his friends and family.
In a Percy Jackson has a Omnitrix fanfiction, the plot could involve Percy being sent on a special mission by Chiron. During this mission, he stumbles upon the Omnitrix. He soon discovers that the Omnitrix has some sort of connection to the Greek gods. As he uses it to transform into various alien forms, he finds that each form has a unique ability that can either help or hinder him in his quest. For example, one form might be able to swim super fast in the ocean, which is great for underwater battles, but another form might be weak against fire, which could be a problem when facing a fire - breathing monster. Along the way, he has to learn to balance his use of the Omnitrix with his own demigod skills and knowledge of Greek mythology to succeed in his mission.