Another antagonist could be a dark force that wants to corrupt Naruto's powers. Since he has the powers of Kami and Zeus, this dark force could see him as a powerful vessel. It could be an ancient evil that has been dormant and now awakens to try and take over Naruto's body and use his powers for its own malicious purposes.
One possible antagonist could be those in the ninja world who fear Naruto's new - found power. They might see him as a threat to the existing power structure. For example, some of the more traditional ninja clans could be against him.
Well, it could be about Naruto having extraordinary powers due to his divine parentage. Maybe he can control both ninja techniques and godly powers from Kami and Zeus. He might face challenges from both the ninja world and the realms of the gods.
Hera could be a main antagonist. Since she's often jealous and protective of Zeus, she might not be happy with Percy being adopted. She could try to undermine him at every turn. Another could be Ares. He might see Percy as a threat, especially if Percy starts to gain more power as Zeus' adopted son.
Some of the minor gods who feel overshadowed by Zeus and Hera might be antagonists. In this fanfiction, they could see Percy as a symbol of the over - dominance of Zeus and Hera. They might plot against Percy to send a message to his parents. These minor gods could use their unique powers and their knowledge of the magical world to create all sorts of problems for Percy, from cursing him to setting up traps in his path.
The village elders could be antagonists. They might see Naruto as a threat due to his devil - like origin and try to eliminate him. They fear that his powers could bring chaos to the village.
The Akatsuki could still be the main antagonists. They might see Naruto's new connection to Hades as a threat or a powerful tool they can use. They would try to capture him to either control his powers or eliminate him.
It could be Orochimaru. Since Orochimaru is always after powerful ninja and their abilities, if Naruto is now the son of Asuma, he might see Naruto as a new target with unique potential.
Another possible antagonist could be a rogue Titan from the Titan world who somehow makes its way into the Naruto universe. This Titan could be enraged or corrupted and see Naruto as an enemy because of his mixed heritage. It could cause destruction across the ninja world, and Naruto would have to stop it using his ninja skills and Titan powers.
One possible antagonist could be those who oppose the mixing of the Naruto world and the Greek mythology elements. They might see Naruto as an abomination because of his unique heritage as the son of Demeter. Another could be some of the traditional Naruto villains who now see Naruto as an even greater threat due to his newfound powers related to Demeter. For example, Orochimaru might be interested in Naruto's powers for his own evil experiments.
One possible main antagonist could be a rogue ninja who was once an apprentice of Orochimaru and is jealous of Naruto's connection to the Sannin. This ninja might want to steal Naruto's new - found powers related to the Sannin. Another could be a hidden organization that sees Naruto as a threat because of his potential with the Sannin's abilities. They fear he will disrupt their plans for power in the ninja world.
One possible antagonist could be a jealous Roman god who doesn't want Naruto, as the son of Jupiter, to gain too much power. Maybe Pluto, who might see Naruto as a threat to his own domain in the afterlife.