Some of the more power - hungry and jealous demigods could be antagonists. They might not like that Percy has been chosen as Nyx's champion and has access to new powers. They could plot against him, create obstacles in his quests, and spread rumors to turn others against him.
There could be ancient creatures that are enemies of Nyx. These could be awakened by Percy's new role as her champion. They would oppose Percy and Nyx, seeing them as a threat to their own existence. These creatures could have powers that are difficult for Percy to counter, and they might lurk in the darkest corners of the mythological world, waiting to ambush Percy at every turn.
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.
Other demigods who are power - hungry could also be antagonists. They might envy Percy's new connection to Nyx and try to undermine him at every turn. They could form alliances against him and plot to steal his powers or his status as the heir.
Some of the more powerful and vengeful spirits in the Underworld itself could be antagonists. They might not want an outsider like Percy to be the Champion of Hades. They could try to undermine him at every turn, like spreading false rumors about him among the other Underworld denizens or directly attacking him when he's vulnerable during his quests in the Underworld.
The Titans could still be antagonists. They might see Percy as a threat in his new role as the champion of Hecate. Another could be some rogue demigods who are jealous of Percy's new power and want it for themselves. And there could be magical creatures that have been corrupted by dark magic and oppose Percy's efforts to bring balance with Hecate's magic.
One possible plot for a 'Percy Jackson Champion of Nyx fanfic' would be that Nyx chooses Percy as her champion to stop an ancient evil that is rising. Percy would have to train in the ways of Nyx, which are very different from what he's used to. He'd have to learn to harness the power of the night, deal with Nyx's shadowy minions, and find a way to unite the demigods who are skeptical of his alliance with Nyx. As he delves deeper into this role, he uncovers secrets about Nyx's past and how it ties to the future of the demigod world.
The Titans could be antagonists. Since Nyx is an ancient and powerful goddess, the Titans might see Percy as a threat because of his connection to her. They could try to either turn him against Nyx or simply eliminate him.
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.
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.
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.