Another possible antagonist could be an ancient monster that was once sealed away by Hestia. Now that Percy is her champion, the monster sees an opportunity for revenge. It could be a powerful and intelligent creature that uses its knowledge of Hestia's past to try and defeat Percy. It might target the places and people that Hestia holds dear, forcing Percy to constantly be on the defensive to protect them.
Perhaps some rogue demigods who have turned against the gods and don't believe in Hestia's values. They could be jealous of Percy being chosen as her champion. They might try to undermine his efforts and steal any power or artifacts related to Hestia that Percy is protecting. They could form a sort of anti - Hestia group, causing trouble for Percy at every turn.
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.
The Furies could also be major antagonists. They are already powerful and feared in the Underworld, and Percy as the Champion of Erebus might be seen as a threat to their authority. They could try to undermine him at every turn, setting up traps and sending various minions after him. Additionally, there could be a new, unknown force emerging from the darkest parts of Erebus that is opposed to Percy's role. This could be a primordial being that has been awakened by Percy's ascension and wants to return the Underworld to a more chaotic and lawless state.
Maybe some rogue demigods who are power - hungry. They see Percy's new title as Champion of Ouranos as a way to gain ultimate power. So they would try to either take the title from him or prevent him from using his powers. They could form a secret group and plot against Percy, using their own demigod abilities and knowledge of the demigod world to their advantage.
The obvious antagonists could be the monsters that are constantly trying to thwart Percy's plans. For instance, the Minotaur could make a comeback, being more powerful and cunning this time. It might be under the control of a more powerful force that doesn't want Percy to succeed as Hera's champion.
The children of Ares might also act as antagonists. They are often at odds with the followers of Aphrodite in the Percy Jackson universe. They might see Percy being the champion of Aphrodite as a threat to their own power and influence. So, they could constantly try to pick fights with Percy, both physically and by trying to turn other demigods against him.
One main antagonist could be Nyx, the goddess of the night. Since Apollo represents the sun, Nyx might see Percy as a threat as he is the champion of Apollo. She could try to plunge the world into eternal darkness and send her monstrous children to stop Percy.
Well, there could be some ice - related monsters or creatures that serve Khione. For example, ice giants or frost - breathing serpents. These could be constantly attacking Percy or trying to prevent him from achieving his goals while he's Khione's champion. Also, there might be some jealous demigods who think Percy is getting special treatment from Khione and they become antagonists by trying to sabotage him.
Hestia could use her power over the hearth to protect Percy and his friends. For instance, when they are in a dangerous situation, she might create a protective barrier around them using the warmth and energy of the hearth.
The main antagonists could be the Titans who are still resentful of the gods and want to disrupt Demeter's power. Since Demeter is in charge of fertility and the harvest, the Titans might try to cause chaos in that area. Another option could be some dark nature spirits that have been corrupted and oppose Demeter's rule over nature.
Well, a plot could involve Percy going on a quest specifically for Hestia. He might have to find a special artifact that is related to her. Along the way, he could meet new characters who are also loyal to Hestia. He would face challenges that test his loyalty to Hestia and his understanding of her values. For example, he might have to choose between saving the flame and saving his friends, and his decision would show his growth as a champion of Hestia.