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.
A powerful and jealous minor god or goddess of love could be an antagonist. This character might feel that Aphrodite is overstepping by choosing Percy as her champion and decides to take matters into their own hands. They could use their own powers of love in a more devious way, like creating false loves and using them to trap Percy or those he is trying to protect, all in an attempt to stop him from fulfilling his role as the champion.
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.
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 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, one possibility could be that Percy is chosen by Aphrodite for a special mission. Maybe he has to use the powers of love and charm in a unique way. For example, he might need to convince a group of warring mythical creatures to make peace by appealing to their feelings. He could find himself in situations where his normal combat skills aren't enough and has to rely on Aphrodite - given abilities like enhanced persuasion.
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.
Some of the old monsters from Greek mythology that have a vendetta against Aphrodite's lineage could be the main antagonists. Maybe a spurned former lover of Aphrodite who was turned into a monster. This monster targets the daughter of Aphrodite and Percy because they are symbols of Aphrodite's power and influence. It constantly tries to harm them and thwart their plans.
The Titans could be antagonists. They might see Percy as a threat since he's Nyx's champion. They could try to stop him from fulfilling Nyx's tasks or even try to turn him against Nyx.