It could be Orochimaru. He's always been interested in powerful bloodlines, and the Uzumaki's power would be no exception. He might try to do something to Naruto that would piss off an Uzumaki, like trying to steal his chakra or use him for some evil experiment.
Maybe some rogue ninja group that doesn't respect the Uzumaki clan. They could target Naruto because of his Uzumaki heritage, thinking they can easily take him on. But they soon find out that pissing off an Uzumaki, especially Naruto, is a very bad idea. They could be after the secrets of the Uzumaki's sealing jutsu or just be jealous of their power.
Well, one key element could be the strong and unique nature of the Uzumaki clan. They are known for their vitality and powerful chakra. In this fanfiction, it might focus on how Naruto, being an Uzumaki himself, has certain traits that make him formidable. Another element could be the situations or characters that might potentially 'piss off' an Uzumaki, like enemies underestimating their power or messing with their loved ones.
The Akatsuki could also be antagonists in this fanfiction. They might see the Uzumaki bloodline as a threat or as a valuable asset to their goals. So they would target Naruto not just as a jinchuriki but also as the heir of the Uzumaki clan.
A new character entirely could be the main antagonist. For instance, an alien - like being from a far - off land who comes to the ninja world with the sole intention of destroying it. Naruto, being the strongest ninja, stands in their way and ultimately pays the price with his life.
Perhaps an outside force that manipulates the Young Justice team into going against Naruto. This could be a villain who wants to create chaos in Naruto's world. The Young Justice, being unaware of the manipulation, start acting in ways that piss Naruto off. In this case, the real antagonist is the behind - the - scenes villain, but the Young Justice members end up being the ones Naruto has to deal with directly, making them sort of antagonists in Naruto's perspective.
It could be someone who is jealous of their new - found bond. Maybe a rival of Uzumaki who doesn't want him to have such a powerful ally in his sister.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
Maybe a new character could be introduced as the main antagonist. This character could be from a hidden village that has been secretly observing Naruto's growth. They realize his potential as a weapon and plan to steal his power or use him to start a war against other villages.
Krampus himself could be the main antagonist. Since he is a figure known for punishing the naughty, his actions in the Naruto world might be seen as unjust. He could be going after ninjas without really understanding their motives, causing a lot of trouble for the main characters.
It could be a rogue ninja who also has somehow acquired bankai - like powers. This antagonist might be jealous of Naruto's newfound abilities and wants to prove that he is the stronger one. So he starts causing chaos in the ninja world, challenging Naruto at every turn.
It could be a rogue ninja who wants to disrupt Naruto's business to gain power. Maybe he's jealous of Naruto's success and popularity in the business world. He could try to steal Naruto's business secrets or sabotage his operations.