A group of Titans who were once defeated but have now regained some of their power could be the antagonists. They might resent Apollo for his role in their previous defeat and target Percy as a way to get back at the god. They could use their old and powerful magic to create all sorts of obstacles for Percy.
The Python, which was once a foe of Apollo, could be resurrected in the fanfiction. It could see Percy as a symbol of Apollo's resurgence and try to kill him. The Python has the power to cause great destruction and its venom could be deadly even to a demigod like Percy. It would be a very formidable antagonist for Percy to face in his role as Apollo's champion.
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.
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.
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.
Another possible antagonist could be some of the more power - hungry and jealous demigods. They might not like that Percy has been adopted by Apollo and is getting special attention. They could try to sabotage his missions or spread rumors about him to turn the other demigods against him.
One possible plot could be Percy Jackson being chosen by Apollo to complete a special quest. Maybe he has to retrieve a lost artifact of Apollo that is crucial for maintaining the balance between the gods. Along the way, he might face various monsters and challenges, like fighting against the minions of a dark god who wants to use the artifact for evil purposes.
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.
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.
Apollo himself could be seen as an antagonist in a way, because of his possessive behavior towards Percy. His actions that restrict Percy's freedom and independence are antagonist - like. However, in some cases, other characters who oppose Apollo's possessiveness might also be antagonists from Apollo's perspective. For example, if Percy has a close friend who tries to protect him from Apollo, Apollo might view that friend as an enemy.