Who could be the main antagonist in a 'naruto earthbound immortal fanfiction'?

It could be Orochimaru. Since he has always been obsessed with immortality and power, he might see Naruto's new - found immortality as something he wants to steal or study. He could use his dark jutsus and his army of followers to try and capture Naruto.

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 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.

The Akatsuki could be the main antagonists. They might see the immortal human as a valuable asset or a threat to their plans. For example, they could want to use the immortal human's unique abilities for their own ends, like using their immortality to perform dangerous and unethical experiments. Or they could view the immortal human as a threat to their goal of world domination, fearing that the immortal human could rally the ninja world against them.

A new and powerful Titan could be the main antagonist. This Titan could be jealous of Percy's immortality and see it as something that should belong to Titans instead. He could gather a new army of monsters and try to overthrow the Olympian gods once again, with Percy being the biggest obstacle in his way.

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.