Lex Luthor could also be a possibility. He could be interested in the technology and powers that both Black Widow and Wonder Woman possess. He might try to manipulate them into fighting each other so he can study them and use their powers for his own evil purposes.
Ares could be a great antagonist. He might see the union of Black Widow and Wonder Woman as a threat to his plans of chaos and war. His god - like powers would pose a significant challenge to the two heroines.
Cheetah could be an antagonist. Jealous of Wonder Woman's new alliance with Black Widow, she could team up with some underworld forces. Her speed and ferocity, combined with the resources of the underworld, would create a tough obstacle for Black Widow and Wonder Woman to overcome.
The Green Goblin could also fit well. He could form an alliance with some of Wonder Woman's foes. His chaotic nature and love for destruction would contrast with the heroes' quest for peace. He could target both of them, trying to break their partnership and cause mayhem in the city.
A possible main antagonist could be a new character created specifically for this fanfiction. A scientist who has somehow managed to combine Greek magic with Hydra technology. This character could be creating powerful hybrid creatures that are causing havoc in both Percy's world and the Marvel universe. Percy and Black Widow would need to put aside their differences and work together to stop this mad scientist.
Lex Luthor could be a great antagonist. He could be jealous of the new alliance between Wonder Woman and Batman and see them as a threat to his own plans for world domination. He might use his intellect and resources to try and turn the public against them or create some high - tech weapons to take them on.
The Cheetah could be a good choice too. She could be working with some high - tech criminals to steal some powerful artifacts. Her speed and ferocity combined with the new allies she has made could put Batman and Wonder Woman in a tough spot as they try to stop her from achieving her evil goals.
It could be an evil ninja from Naruto's world who has somehow gained the powers of a DC super - villain. For instance, Orochimaru could merge with Lex Luthor's intelligence and resources. He would be a formidable foe as he combines Orochimaru's snake - like abilities and knowledge of jutsu with Luthor's strategic mind.
A powerful and corrupt military general could be a great antagonist. He could be using his position to traffic illegal weapons and is protected by a network of corrupt officials. The Punisher would be after him for his immoral actions, and Black Widow would be involved because it's a threat to international stability.
Circe is another option. She has the power to transform people and cause chaos. In the fanfiction, she could be jealous of Wonder Woman's beauty and power and Percy's connection to the gods. She might try to turn them against each other by using her magic on them or on those around them, causing misunderstandings and battles.
The Green Goblin could be a likely villain. His chaotic nature and powerful glider - based attacks would pose a great threat to both Wonder Woman and Spider - Man. He could be after some sort of powerful artifact that Wonder Woman has knowledge of, and Spider - Man gets involved trying to stop him from getting it.
Maybe it's an ancient evil entity that has been awakened by the combined power of Naruto's chakra and Wonder Woman's divine energy. This entity sees them as a threat to its plan of ruling both universes and thus tries to eliminate Wonder Woman first, which is when Naruto steps in to save her.
One common theme is female empowerment. Both Wonder Woman and Black Widow are strong female characters, and in fanfiction, they often team up to show the power of women working together. They break gender norms and prove that they can be just as capable, if not more, than male superheroes.