One great fanfiction idea could be a story where Zeus, Poseidon and Hades have to team up to face a common threat that is even more powerful than them. Maybe a primordial being awakens and they must put aside their differences. In this story, Percy could be the mediator between them, using his unique position as the son of Poseidon to try and keep the peace among the three powerful gods.
You can start by giving her a distinct personality. Maybe she's shy because she's always been feared for her father's sake. Then, gradually show her growing confidence as she discovers her own strengths. For example, if she has the power to raise small skeletons as minions, at first she's scared of it, but later she learns to use it for good. Also, explore her relationships with other characters. Her friendship with Percy could be based on mutual understanding of being different among the demigods.
A fanfiction could be about a competition among Zeus, Poseidon and Hades, and Percy has to be the judge. The competition could be something like who can best help a small, struggling mortal town without using their full power. This would show a different side of the gods and also test Percy's ability to be fair and objective.
The main characters are Percy and Fem Hades. Percy is the protagonist from the original series, and in the fanfiction, his story continues with his interactions with Fem Hades. Fem Hades, on the other hand, is a unique take on the Hades character. She could have different personality traits compared to the male Hades in the original mythology. There might be other characters that are introduced as the story progresses, but these two are the foundation of most Percy Jackson and Fem Hades fanfiction.
One great fanfiction could be 'The Hidden Path in the House of Hades'. It explores the relationship between Percy and Annabeth in more depth during their time in the House of Hades. The author adds some really interesting side quests that the characters embark on, which makes it a thrilling read.
One great 'Percy Jackson Hades Self Insert Fanfiction' is 'Into the Underworld'. It really delves into the character's perspective as they are inserted into Hades' world. The author does a great job of showing how the rules and power dynamics of the Underworld work from an outsider's point of view. It also has some interesting interactions between the self - insert character and Hades himself, which makes for an exciting read.
Annabeth Chase might also be an ally. Her intelligence and strategic thinking could be valuable assets for the son of Hades. She could help him plan his moves, especially when dealing with the complex politics among the demigods and gods.
Maybe he faced an extremely powerful enemy that he couldn't overcome. It could be a new and terrifying monster created by the Titans or some other dark force.
Hades' character could be developed by showing his more paternal side towards Percy. He might start as a cold and distant figure but gradually show concern and protectiveness. For example, he could save Percy from a dangerous monster in the Underworld.
One possible plot could be Percy struggling with the duality of his parentage. He might face discrimination from both the children of Hades and Poseidon. For example, the children of Poseidon could see him as a threat because of his connection to Hades, and vice versa. He could then go on a journey to prove that his dual heritage doesn't define him, but rather gives him unique powers to protect the world of the gods and mortals.