One plot idea could be Percy having to use Hephaestus' skills to repair a powerful, ancient weapon that is the key to saving the demigod world from a new threat. Another could be Percy competing in a Hephaestus - inspired crafting contest among the demigods to prove his worth as the champion.
His relationship with Annabeth might change. She could be both impressed and a bit jealous of his new skills related to Hephaestus. Percy would now have an area of expertise that she doesn't fully understand. With the Hephaestus kids, he might start as an outsider but gradually earn their respect as he proves himself in tasks related to their father's domain.
I think Tyson, Percy's cyclops half - brother, would be a good character. His strength and loyalty would be assets in any situation. Also, Nico di Angelo could be in the fanfic. His control over the dead could provide an interesting dynamic, especially if they are exploring some dark or ancient places related to Hephaestus' past works. And of course, Clarisse La Rue. Her combat skills and fierce personality would add some drama and action to the story.
One common theme is Hephaestus' role in helping Percy with his quests. Since Hephaestus is the god of blacksmiths and craftsmen, he might provide Percy with a special weapon or tool. Another theme is the exploration of Hephaestus' relationship with his children in the context of Percy's interactions with them. For example, how Percy might help a Hephaestus - descended character come to terms with their heritage.
Well, one possible plot could be that Percy discovers his new relation to Hephaestus later in his adventures. He might then start to have an affinity for creating and fixing magical items, which could be a new skill for him in battles against monsters. He could also meet other descendants of Hephaestus who teach him more about his heritage.
Well, in fanfiction, if Percy is the Champion of Pontus, his story would be filled with more mystery. Pontus is an ancient and powerful entity. Percy might be called upon to do things that go against the traditional ways of the Olympian - influenced world he's used to. He could discover secrets about the origin of the sea and his own powers that were previously unknown. This would not only make for an exciting read but also show how Percy can adapt and make his own path even when given a very different role.
One possible fanfiction could be about Percy Jackson seeking the guidance of Hestia, the goddess of the hearth, who has now become a champion in some way. Maybe she is a champion against a new threat to the demigod world. Percy could learn new skills or gain unique powers from her. For example, powers related to home and family, which could be used to protect Camp Half - Blood in a different way than before.
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.
In some fanfiction, Percy Jackson might be depicted as a champion of Kronos, but this goes against his character in the original series. In Rick Riordan's official books, Percy is a hero who stands against Kronos and his evil plans.
In Percy Jackson fanfiction, the 'Champion of the Sun' could be a character with special powers related to the sun. It might be someone who has the ability to control sunlight, perhaps for combat or to perform other extraordinary feats. Maybe they are a hero figure within the fan - created story, with a backstory that ties them to the sun in some way, like being blessed by a sun - related deity.