Maybe there could be an Asgardian traitor who doesn't like the idea of a half - human being part of the royal family. This traitor could team up with some of Naruto's enemies from the ninja world to create chaos. They could plot to steal Naruto's powers or his special weapon if he has one, and Naruto has to stop them to protect both Asgard and the ninja world.
The main antagonists could be the Akatsuki from the Naruto world. They might see Naruto's new powers as a result of being Sparda's son as a great asset and try to capture him to use for their own evil plans, like creating some sort of super - weapon or using his power to take over the ninja world.
Well, perhaps there could be a powerful underwater clan that sees Naruto as an abomination because he is a half - human, half - Neptune's son. They view his existence as a threat to the balance of the ocean realm and the ninja world. They send their best warriors to stop Naruto from growing in power and spreading his influence. Also, a mad scientist - type character who wants to experiment on Naruto to unlock the secrets of his Neptune - heritage could be an antagonist.
It could be the people in the village who are against Naruto because of his new identity as the son of Manda. They might see him as a threat to the village's stability.
The main antagonists could be those who were once enemies of Obito. They might see Naruto as a threat simply because of his relation to Obito. For example, some of the old - school ninja clans who still hold grudges against Obito's actions in the past could target Naruto.
It could be those who fear Naruto's power as the son of Lilith. For example, some of the more traditional ninja clans might see him as a threat to their power and status in the ninja world.
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.