One idea could be that the OC, as the son of Hades, struggles with the stigma associated with his father's realm. He could be trying to prove that he's not all about death and darkness. For example, he might use his powers to help the living in secret, like saving people from disasters that are on the verge of happening.
One idea could be Percy having to navigate the Underworld politics due to his relation to Hades. He might face challenges from other underworld creatures who don't trust him. Another could be Percy using his newfound powers as the son of Hades to fight against an ancient evil that threatens both the mortal and underworld realms. Or perhaps he has to deal with the stigma of being Hades' son in the demigod community and prove himself worthy.
Another plot could involve Percy's relationship with his father, Hades. Maybe Hades has some grand plan for Percy that he doesn't fully understand at first. Hades could be trying to use Percy to gain more power in the world of the gods, but Percy resists and has to fight against his father's wishes while also dealing with the internal conflict of not wanting to completely turn his back on his family.
Another plot could be Percy discovering a long - lost relative among the children of Hades. This relative has some sort of power or knowledge that could change Percy's understanding of his own identity and his place in the demigod world. It could lead to a journey of self - discovery for Percy as he navigates this new family connection within the dark and often misunderstood realm of Hades.
A plot could revolve around the son of Hades having to face a new and powerful monster that is specifically targeting him because of his heritage. This would lead to a series of adventures where he has to use his unique powers from Hades to survive and protect his friends. For example, he might be able to control the undead in new and unexpected ways to fight this monster.
A key trait could be his sense of isolation. Growing up as the son of Hades, he's likely to be shunned by others. This could make him a very introverted character, but also one with a deep sense of self - awareness as he's had a lot of time to think on his own.
One interesting plot idea could be her struggling with the stigma associated with being Hades' daughter. For example, other demigods might be scared or distrustful of her at first. Another could be her going on a quest related to the Underworld, like trying to stop some escaped souls from causing chaos on Earth. She could also have a special power related to the dead that she has to learn to control, such as being able to see the last moments of a dead person's life, which could be both a gift and a curse as she might see some really traumatic things.
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 idea could be Percy having to deal with the stigma of being associated with Hades in the demigod world. He might face distrust from other demigods who are scared of Hades' reputation.
One plot idea could be Harry discovering his true parentage and having to deal with new powers that come with being Hades' son. He might struggle to control the power of the underworld, like being able to summon the dead or manipulate shadows.
One plot idea could be Percy's struggle to fit in. Since his parentage is so unique, he might not be fully accepted by either the children of Hades or those of Hestia. Another could be his discovery of new powers. Maybe he can control both the underworld flames (associated with Hades) and the hearth flames (from Hestia).