One possible main antagonist could be a rogue ninja from a forgotten clan who has a vendetta against the Land of Fire. They might be seeking revenge for something that happened in the past and see Naruto as the symbol of the new regime they want to topple.
Perhaps an ancient evil entity that was sealed away long ago in the Land of Fire. Naruto's rise as the Lord awakens this entity, which sees the land as its rightful domain. It uses dark jutsu and mind - control on some of the villagers to create chaos and tries to drive Naruto out so it can rule over the land once again.
It could be a group of corrupt politicians within the Land of Fire itself. They oppose Naruto's leadership because they fear that his reforms will cut into their own power and wealth. So they plot behind the scenes to undermine him at every turn, and they might even hire mercenary ninjas to carry out their dirty work.
One possible fanfiction could be about the feudal lord's role in maintaining peace during a time of political unrest in the Land of Fire. Maybe he has to deal with various ninja clans vying for power and uses his diplomatic skills to keep the balance.
Well, one possible plot could be Naruto using his unique skills and charisma to bring about great reforms in the Land of Fire. He might focus on improving the living conditions of the common people, building better ninja academies in every village, and strengthening the bonds between different clans.
It could be some powerful local warlords who are exploiting the resources of the Land of Wave. They might have their own private armies of ninjas and use fear to keep the people in check.
It could be that the very concept of the hellish order itself is the antagonist. If Naruto wants to bring a more positive change, the established rules and the entities that enforce them would be against him. For instance, the guardians of the different levels of hell might not want Naruto to interfere with their way of doing things and would fight to keep the status quo.
The feudal lord could be secretly training as a ninja himself. He wants to better understand the world of the ninja and be able to protect himself without relying solely on his guards. This would add an element of mystery as he has to keep it a secret from his court.
Perhaps an old enemy of the Fire Daimyo, like a corrupt noble or a power - hungry general within the Fire Country who sees Naruto as a threat to their plans of overthrowing the Daimyo. So they try to eliminate Naruto, but the Fire Daimyo intervenes to save him.
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.
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.