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.
It could be Danzo. He was always power - hungry and secretive. He might see the immortal human as a threat to his control over Konoha. Danzo would likely try to either eliminate the immortal human or use their immortality for his own gain. He could manipulate the situation to make the other ninja think the immortal human is a danger, all while planning to harness their power for himself. He might even use his root anbu to spy on and attack the immortal human.
It could be an earthbound immortal who has been corrupted by dark chakra. This immortal might have once been good but something in the past made it turn evil. It now wants to take over the ninja world and use its power to spread darkness.
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.
" Immortal Human Emperor " was an Eastern fantasy novel that was published on Miracle Network. The author was Ironback Cockroach. It mainly told the story of the young Qin Xuan who accidentally obtained a mysterious stone bead and cultivated a supreme inheritance. From then on, he embarked on the path of invincibility and advanced triumphantly. He swept through the nine heavens and ten lands and became the immortal emperor. The plot of the novel was full of ups and downs, and the characters were vivid and well-liked by readers. While waiting for the TV series, he could also click on the link below to read the classic original work of " The Legend of Mortal Cultivation "!
It could be another ghost who is jealous of Casper's new human form. This ghost might try to sabotage Casper's attempts to fit into the human world, perhaps by causing chaos around him and making humans afraid of him again.
It could be a vampire who resents Damon for becoming human. This vampire might see it as a betrayal of their kind. They could try to either turn Damon back into a vampire against his will or simply make his human life miserable.
The main antagonist could be a dragon hunter. Since Saphira is now in a human form and has lost her dragon defenses, the hunter sees her as an easy target. He might be after her because he believes that she has some connection to the dragons and he wants to use her to find the dragon lair or something valuable related to dragons.
It could be another monster who is jealous of Sans' new form. This monster might feel that Sans is getting special treatment or opportunities because he's now a human. So, this jealous monster tries to find a way to either turn Sans back into a skeleton against his will or make his human life miserable by causing trouble for him in the human world, like spreading false rumors about him.
Dr. Eggman would be an obvious choice. Since Sonic is now in a more vulnerable human form, Eggman could see this as the perfect opportunity to finally defeat Sonic and take over the world. He might come up with new devious plans, like creating robots specifically designed to target a human Sonic.