Perhaps an organization that wants to use Naruto's rare bloodline for their own evil purposes. They could be experimenting on other ninjas with different bloodlines and see Naruto as the key to their ultimate experiment, like creating an army of super - powered ninjas.
It could be a rogue ninja who also has a rare bloodline and sees Naruto as a threat to his own power and status. This antagonist might want to steal Naruto's bloodline or simply eliminate him to be the only one with such a special ability.
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.
Perhaps a dark entity from another dimension. This entity senses Naruto's powerful bloodline and wants to possess him or use his body as a vessel to enter the Naruto's world and wreak havoc. It would have otherworldly powers that Naruto has never faced before, making it a truly formidable antagonist.
It could be a rogue ninja from a forgotten clan who is jealous of Naruto's new - found power from his hidden bloodline. This antagonist might try to steal Naruto's bloodline ability for himself.
Maybe an ancient evil spirit that was sealed away long ago but is awakened by the emergence of the Adamantium bloodline. This spirit believes that the power of the bloodline can be used to break free from its final seals and wreak havoc on the world.
It could be a powerful ninja from an ancient clan who sees the character with the godlike multi - bloodline as a threat to their own clan's supremacy. This antagonist would be very skilled in traditional ninja arts and would use their knowledge of the old ways to try and defeat the protagonist.
It could be someone who wants to steal the One for All bloodline from Naruto. Maybe a rogue ninja from another village who has heard about this new and powerful ability in Naruto and decides to make it his own.
The main antagonists could be members of an ancient clan who see Naruto's emerging powers from his bloodline as a threat to their own plans for power. They might fear that his newfound abilities will disrupt the balance of power they have been carefully cultivating for generations.
It could be Orochimaru. He is always interested in new and powerful weapons. With Naruto having hidden blades, Orochimaru might see it as a threat and try to take the blades for himself, leading to a conflict.
Perhaps the main antagonist is a dark figure from within Konoha itself. This person doesn't want Naruto's family to be reunited because it could expose some dark secrets about their own actions in the past. They would do anything to keep the sister hidden and Naruto in the dark, even if it means causing harm to both of them.
It could be a rogue ninja who was once part of the same clan that the hidden blade belongs to. He wants the blade back to gain ultimate power and is willing to do anything to get it from Naruto.