Darkseid is another potential foe. He could be interested in the son's unique heritage, wanting to either corrupt him or use his powers for his own evil purposes. Given Darkseid's power, the son would have to rely on both his Batman - like wits and Wonder Woman - like strength to oppose him.
Circe could also be an antagonist. She might be jealous of the son's connection to Wonder Woman and Batman. She could use her magic to create illusions and manipulate situations to turn the son against his parents or make him doubt himself.
Lex Luthor could be a main antagonist. He might see their union as a threat to his power and schemes, and try to break them apart or use their relationship against them.
The main antagonists could be the Titans who see Percy as a threat because of his combined powers. They might try to stop him from fully realizing his potential as the son of Wonder Woman.
There could be new villains from the cosmos. Since Naruto is the son of Galactus, other cosmic entities who oppose Galactus might target Naruto. For example, some powerful alien race that wants to get rid of any trace of Galactus' bloodline. They could send their most powerful warriors to the ninja world to take on Naruto.
One potential antagonist could be Orochimaru. He is always after power and the power of the Nibi within Naruto would be very appealing to him. He might try to capture Naruto to extract the power or use him for his own evil experiments.
There could be new antagonists created specifically for this fanfiction. Maybe some other ninja from a far - off land who are jealous of Naruto's power. They could come to the Hidden Leaf Village to challenge Naruto and steal his speed - related secrets.
One interesting storyline could be the son's struggle to balance the very different legacies of his parents. He might have Batman's brooding nature and tactical mind, while also having Wonder Woman's sense of justice and connection to the gods. This internal conflict could lead to some great character development.
Well, in such a fanfiction, the son could inherit amazing powers. Maybe he has Batman's strategic mind and Wonder Woman's super strength. He could use his skills to fight new villains in Gotham or Themyscira.
One idea is that their son would be a bridge between two different worlds. He would grow up with the values of both Batman and Wonder Woman. In the fanfiction, he could face challenges that neither of his parents faced. For instance, he might have to deal with the politics between the Amazons and the humans. His powers and skills would be put to the test as he tries to bring peace and understanding between these two groups. He could also form new alliances, using his unique heritage to his advantage.
Perhaps there's a dark entity that was once an enemy of Achilles in the original mythology. And now that Naruto is the son of Achilles in the fanfiction, it targets him. This entity could have powers that are specifically designed to counter Naruto's Achilles - related powers, making for some intense battles.
She could be devastated. Wonder Woman might go into a period of mourning, isolating herself from others as she grapples with the loss. She was used to having Batman as an ally, and his death would leave a big hole in her life.