Perhaps a rogue shinobi from another village who believes that the avatar of death is a sign of doom for all ninja villages. He tries to kill Naruto to prevent the so - called 'curse' from spreading, but in reality, he's just power - hungry and wants to be the one to control the power if Naruto were to die.
It could be an ancient dark ninja who wants to use Naruto's new 'avatar of death' powers for his own evil purposes. This ninja might have been researching death - related jutsus for years and sees Naruto as the key to his ultimate power.
Another possibility is a powerful spirit from the Avatar world that has been corrupted by the introduction of Naruto - like powers. This spirit could resent the intrusion of this new power source and seek to destroy anyone associated with it. It might have unique abilities that combine the spiritual magic of the Avatar world with the raw power of Naruto's chakra, making it a formidable foe.
It could be a rogue ninja who has somehow harnessed the power of the Crimson Avatar in a dark and corrupted way. This ninja could use his powers to try and take over the ninja villages, believing that the traditional ninja ways are weak. He might see himself as a new kind of leader, using fear and his overwhelming Crimson Avatar powers to rule.
In the 'immortal naruto avatar fanfiction', the main antagonist might be a corrupted Avatar. This Avatar, instead of maintaining balance, wants to use Naruto's immortality to become an all - powerful dictator. He or she would have the bending abilities of an Avatar and the knowledge of how to use them in devious ways. This would put Naruto in a difficult position as he has to either stop the Avatar or find a way to redeem him/her while also protecting his own immortality.
It could be a new, powerful ninja who has a vendetta against Naruto. This antagonist might be jealous of Naruto's popularity and power, and wants to take over the Hidden Leaf Village.
A rival medicine cat from another Clan might be the main antagonist. They could see Alderheart as a threat to their own status or the well - being of their Clan. So they might use dark magic or some underhanded tactics to bring about his death.
A dark and mysterious sorcerer could also be the main antagonist. The sorcerer has a long - standing grudge against Byakuya's family or the place he protects. Using dark magic, the sorcerer sets up a series of deadly traps and curses, which ultimately lead to Byakuya's death.
A secret organization that has been operating in the shadows and views Naruto as a major threat to their plans. They might be planning something big like taking over the ninja world, and Naruto's presence and influence stand in their way. So, Naruto fakes his death to gather intelligence on this organization without alerting them.
It could be Orochimaru. Imagine if he also found a way to cross over into the Avatar world. With his knowledge of dark jutsu and his desire for power, he would be a formidable antagonist. He might try to use the bending abilities for his own evil purposes and also target Naruto because of his immortality, seeing it as something he could steal or use to further his plans.
It could be Orochimaru. Even though he's had his moments of redemption, he might relapse into his old ways. He could see Naruto's 'death' as an opportunity to carry out some evil plan that Naruto would have stopped. Maybe he wants to use Naruto's death to create chaos and then take advantage of the power vacuum in the ninja world.
It could be an evil shinobi who has somehow managed to control or manipulate the God of Death's powers for his own malicious purposes. This shinobi might use the power to create chaos in the ninja world, like raising the dead and making them fight against the living.