In such a fanfiction, Percy might help Hades with some internal underworld issues. For example, dealing with rebellious spirits. Since they are friends, Percy could use his unique abilities and knowledge of the modern world to assist Hades in modernizing certain underworld operations, like record - keeping. It could also lead to interesting interactions with other underworld deities who might be suspicious of this new friendship.
In fanfiction, Percy Jackson as the champion of Hades could bring a whole new dynamic. He might have to deal with the darker aspects of the Underworld. For example, he could be in charge of ensuring the souls are properly sorted, which is a huge responsibility.
Some of his friends might be very wary at first. After all, Hades has a rather dark reputation. But if Percy can show them the good in Hades, they might gradually come around. For example, Annabeth might be initially skeptical but after seeing how Hades helps Percy in some way, she could start to accept the friendship.
One common theme is the exploration of Hades' character. Many fanfictions try to humanize him, showing that he's not just the 'bad guy' as he's often portrayed in the original stories. For example, they might show his love for his family in a different way.
One common theme is the exploration of power dynamics. Since Hades is a powerful god and Percy has his own set of powers, fanfictions often play with how they interact and who has the upper hand in different situations. Another theme is family relations. Hades has a complex family, and Percy's own family situation is also quite intricate. Fanfictions might explore how their family backgrounds affect their relationship and actions.
One idea could be Percy having to mediate between Zeus and Hades during a major feud. Since he's the son of both in this fanfiction, he has a unique connection to them. He could use his powers and his knowledge of both of their domains to try and broker peace.
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.
One common theme is the struggle with identity. Since Percy is usually the son of Poseidon in the original series, making him the Son of Hades in fanfiction often leads to him dealing with a new sense of self. Another theme is power and its consequences. Hades has a lot of dark and powerful abilities, so fanfictions might explore how Percy copes with having access to such powers.
Maybe Percy discovers that there is a conspiracy within the Underworld to overthrow Hades. As the champion, he has to uncover the truth and stop the plot. Along the way, he meets old enemies who now see him as an ally of Hades and try to take advantage of the situation.