Another possibility is that an ancient evil entity that was once sealed away by Naruto's bloodline ancestors is awakened by Naruto's growing power. This entity sees Naruto as a key to either its complete freedom or its destruction, and thus becomes the main antagonist, constantly sending minions or using its own dark powers to try and stop or control Naruto.
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.
Maybe it could be other ninja clans who are jealous of Naruto's newfound sword bloodline. They might see it as a threat to their own power and status in the ninja world.
Some secret organizations that have been manipulating bloodline abilities for their own gain could also be the antagonists. They would not want Naruto to disrupt their plans. They might send their most powerful agents to stop Naruto, leading to intense battles in the fanfic.
Well, the main antagonists could be a group of rogue ninja who are hired by a powerful nation to take down Naruto. Since he is unkillable, they would have to use more underhanded tactics like trying to isolate him from his friends and allies. Or perhaps there is a dark force that is awakened by Naruto's unkillable state, thinking that he is a threat to the balance of the ninja world. This dark force could then become the main antagonist, trying to destroy Naruto at all costs.
Well, in a Naruto fanfiction with an ancient bloodline for Naruto, he could end up having a completely different path in his ninja journey. The ancient bloodline might attract the attention of other powerful entities, either good or bad. For instance, some ancient organizations might try to recruit him, seeing his potential. On the other hand, villains could target him more aggressively to prevent him from reaching his full potential. His training would also be different. Instead of the traditional ninja training, he might have to go through some ancient and secretive training methods related to his bloodline. This could involve meditating in special places or performing rituals that are long forgotten in the ninja world.
Some fanfictions might feature rogue ninja as antagonists. These rogue ninja are after the bounty on Naruto's head or they are trying to steal Jiraiya's knowledge. They attack Naruto and Jiraiya during their travels, making it difficult for them to reach their destination.
Often, it could be the Akatsuki. They are always after the power of the tailed beasts, and with Naruto being protected by the Kyuubi, they would be major antagonists. They might come up with new and devious plans to try and get to the Kyuubi within Naruto.
Maybe a group of dark - type Pokémon that have banded together and are trying to overthrow Arceus. They view Naruto as an easy target at first, but quickly realize he is a formidable opponent with his ninja skills and new Pokémon - related powers. They would constantly try to undermine Naruto and his efforts to protect the Pokémon world.
It could be some rogue ninjas who are against Naruto's new peaceful policies as Hokage. They might see Hinata as a weakness to be exploited to get to Naruto.
The Akatsuki members might also be antagonists. Since they're after the tailed beasts, Naruto with his additional power of lightning control would be a prime target. They might send their most powerful members like Pain or Itachi to deal with him.
It could be a group of rogue ninjas who also ended up on the island. They might be after some valuable resource on the island and see Naruto and Tsunade as obstacles.