It could be an evil shinobi who wants to capture the Kitsune Anbu for their unique powers. Maybe they want to use those powers to take over the Hidden Leaf Village. Another option is a rogue Anbu member who has turned against the village and sees the Kitsune Anbu as a threat to their own plans.
Perhaps the main antagonist is a former ally of the Kitsune Anbu. This person was once part of the Anbu but was corrupted by power. They envy the Kitsune Anbu's abilities and status within the village. So they decide to create a plan to discredit the Kitsune Anbu and gain control over Naruto. They might spread false rumors, set up traps, and even form alliances with other villains to achieve their goals. Their actions would not only put the Kitsune Anbu in danger but also threaten the safety of Naruto and the entire Hidden Leaf Village.
Maybe another kitsune who has gone bad. In the fanfiction world, there could be a rogue kitsune who sees Naruto as a threat. This antagonist kitsune might be jealous of Naruto's power or his connection to the human world. They could try to undermine Naruto's attempts to protect the village or his friends.
It could be another powerful kitsune who wants to use the assassin for their own evil plans. This other kitsune might be jealous of the assassin's skills and wants to control them.
It could be a rogue ANBU member who has gone bad and is trying to expose Naruto's secret identity for some evil purpose.
Perhaps a powerful shinobi from another village who has a special grudge against the Leaf's ANBU. He targets Kakashi and Naruto specifically because of their reputation within the ANBU. He has unique abilities that pose a great challenge to them, like the ability to nullify ninjutsu for short periods.
The main antagonist could be an old - school ninja who believes in the traditional and often harsh ways of the ANBU. They see Naruto's actions as weakening the village and set out to oppose him at every turn, trying to bring back the old ANBU regime.
One potential main antagonist could be an older, power - hungry shinobi who has infiltrated the Anbu in the past. This shinobi could be jealous of Naruto's sudden rise in the Anbu (due to his future knowledge) and sees him as a threat. He then starts to plot against Naruto and the Anbu's new - found positive direction. Or perhaps it's a corrupt politician in the village who uses the Anbu as a tool and doesn't want Naruto to change the Anbu's loyalty from him to the village as a whole.
It could be a rogue Anbu member. This person might be jealous of Naruto's power and popularity within the Anbu. So, they spread false rumors about Naruto which lead to his decision to leave the village. This antagonist would then try to further harm Naruto while he's outside the village, using their own Anbu skills against him.
It could be Orochimaru. He has always been interested in powerful jutsus and Naruto's unique power as a Jinchuriki. He might have developed a new jutsu to make everyone forget Naruto so that he can capture him without interference for his own experiments.
In this fanfiction, Naruto as an ANBU Captain Kitsune could be faced with a powerful and mysterious enemy. He has to draw on his inner Kitsune strength and his knowledge of the ninja world. He might also have to make tough decisions that test his loyalty to the village and his comrades. His Kitsune heritage could give him an edge in battles, with abilities like enhanced speed and the power to manipulate chakra in new ways.
Obviously, Naruto would be one of the main characters. He's the protagonist of the Naruto series and would be the main link to the ninja world in this crossover. Another could be a young Kitsune, maybe one that's just coming of age in the Kitsune world. Their paths cross, and they have to learn to work together.