The Khaos Brigade could be a major antagonist. Their goal of causing chaos would put them at odds with Gohan's sense of justice.
It could be an evil dragon from the DxD lore that decides Gohan is a threat to its plans. Since Gohan is so powerful, the dragon might see him as the only obstacle in its path to rule the DxD world.
If it's a crossover with the One Piece world, Blackbeard could be a potential bad guy. His ability to steal Devil Fruit powers and his ambition to become Pirate King could lead to a clash with Gohan. Gohan's pure - hearted nature would be in opposition to Blackbeard's more devious ways, and their battle could be epic, with Gohan's ki powers against Blackbeard's dark powers.
Perhaps there are dark forces from another dimension. These forces might be attracted to Naruto's power and want to either steal it or destroy him to prevent him from using it against them. They could be powerful demons or sorcerers with their own motives.
Another possibility is that the main antagonists could be the leaders of the other ninja villages. They might view Naruto's godlike and rogue status as a danger to the balance of power among the villages. So, they could form an alliance against Naruto, leading to a lot of political intrigue and conflict in the story.
Well, it could be a powerful gamer who has a grudge against Gohan. Maybe this gamer has lost to Gohan in some important in - game tournaments and now wants revenge. So he devises all kinds of devious plans to undermine Gohan's progress in the game.
In this kind of fanfiction, the Fallen Angels could be main antagonists. They might see Naruto and Sasuke as threats to their plans in the DxD world.
It could be an old enemy of the group who has a personal vendetta against Issei. This antagonist might be jealous of Issei's power or popularity within the group and plots to turn everyone against him. They could spread false rumors or create false evidence to make it seem like Issei has betrayed the group.
The devils who oppose Naruto's presence and his new - found influence in the DxD world could be main antagonists. They might see him as a threat to their power structure and the way things are done in their society.
The devils who oppose Gilgamesh's presence in the Highschool DxD world could be main antagonists. They might see him as a threat to their power and influence in the school.
It could be his own family members who abandoned him. They might come back as antagonists, trying to stop Naruto from reaching his full potential or causing trouble for him out of envy or regret. Maybe they have joined a dark force and are now working against him.
In this fanfiction, the main antagonists might be a group of dark chakra - users. They are jealous of Naruto's godlike powers and want to take over the empire for their own evil purposes. They could be using dark magic to corrupt some of the people in the empire, turning them against Naruto. There might also be some power - hungry politicians within the empire who are not satisfied with Naruto's leadership and want to seize power for themselves. They would plot behind the scenes to bring down Naruto and take over the reins of the empire.