Poseidon could play a regretful father. He may realize too late how much Percy has suffered and then try to make amends. For instance, he might secretly send gifts or powers to Percy to help him defend himself. Hades could be a secret mentor. He has a lot of knowledge about the dark side of the world and magic that could help Percy understand his situation better and find ways to fight back against his abusers.
Poseidon might be presented as a rather powerless god in the context of Percy being abused in fanfic. Despite being Percy's father, he could be constrained by the will of Zeus and other gods, which prevents him from acting decisively to save Percy. Hades, however, might be a more interesting character. He may have his own motives for helping Percy. He could see Percy as a useful tool against the other Olympians or simply as a victim who he can relate to, given his own outcast status among the gods. So, in fanfic, he might play a key role in Percy's journey to overcome the abuse.
One common theme could be the struggle for power. In these fanfics, Percy might be abused as a way to show how the power dynamics among Poseidon, Hades, and the other characters are out of balance. For example, if Poseidon or Hades sees Percy as a threat to their realm, they may abuse him to keep him in check. Another theme could be redemption. Percy, after being abused, might strive to find a way to make things right and gain the respect of the gods again. It could also explore the theme of family. Since Percy is related to Poseidon, the abuse could be a way to show the complex and often strained relationships within the family of the gods.
Writers can start by developing strong characters. For Percy, they need to show his vulnerability as well as his inner strength. For example, they can write about how he copes with the abuse internally while still showing some defiance on the outside. For Poseidon and Hades, they should give them complex motives for the abuse. Maybe Poseidon is being pressured by other gods and takes it out on Percy, or Hades has some long - standing grudge against Percy's family. They can also create a vivid setting. A dark and foreboding Underworld for Hades' part of the story, or a stormy and unpredictable sea for Poseidon's influence. This can enhance the mood of the abuse and Percy's struggle.
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.
Well, it could be about Percy and Poseidon having some sort of adventure together. Maybe they team up to face a new threat in the mythological world. Since it's a fanfic, the author might also explore their relationship in ways not shown in the original stories, like Poseidon teaching Percy some unique powers or skills.
In a 'Percy meets Poseidon' fanfic, Percy might seek advice from Poseidon. Maybe he's facing a new and powerful enemy in the demigod world, and Poseidon could offer some godly wisdom or a special weapon to help him. For example, Poseidon could give Percy the power to control a new aspect of the sea, like the ability to summon sea monsters that are loyal only to Percy.
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.
Poseidon might be extremely angry. He could immediately seek revenge on the abuser. After all, Percy is his son and he has the power to protect him.
Well, it could focus on the political implications. In Greek mythology, the gods often have power struggles. Their marriage could be a strategic move. Poseidon and Hades might join forces to gain more influence among the other gods. Maybe Zeus would be threatened by their union. This could lead to a series of events where they have to prove their loyalty to each other and their new combined rule while facing opposition from Zeus and his allies.
One possible plot could be that Poseidon has to keep the female Percy hidden from the other gods because she's a girl and not the expected male hero. He might train her in secret on an isolated island, teaching her about her powers and the history of the gods.