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.
One possibility is that Naruto gets caught in a powerful genjutsu that makes everyone forget him. He might then struggle to find a way to break the genjutsu on his own. He could start to question his own identity and purpose, as being an Anbu, his very existence in the shadows becomes even more lonely without anyone's recognition.
One option for the main antagonist could be a secret organization that has infiltrated Konoha. They target the Anbu prodigy because they fear the prodigy's abilities could expose their existence and plans. This organization might have members with unique and dangerous abilities themselves. They would constantly send assassins after the prodigy and try to create chaos in Konoha to keep the Anbu and the prodigy distracted while they carry out their real objectives.
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.
It could be a rogue ANBU member who has gone bad and is trying to expose Naruto's secret identity for some evil purpose.
Another possibility is that the antagonist could be a dark - side version of Naruto himself. The discovery of the forgotten bloodline could stir some dark desires within him. This internal struggle could be personified as a separate entity that tries to take over Naruto and use the power of the bloodline for evil. Naruto would then have to fight against this internal darkness as well as external threats who are also interested in his new - found power.
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 new character who has the ability to manipulate people's minds. This character could turn Naruto's friends and the villagers against him by making them believe false things about Naruto. So, this mind - manipulator would be the main antagonist as they are the cause of Naruto's heartbreak.
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.
Maybe it's an evil organization that wants to isolate Naruto. By making his family forget him, they hope to break Naruto's spirit and prevent him from becoming a powerful force in the ninja world. So they target his family's memories as a way to get to him.