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.
Maybe an experiment gone wrong from a hidden research facility in the Naruto world. Scientists were trying to combine the powers of the ninja with the Crimson Avatar energy, but it created a powerful and malevolent being. This being has no loyalty to any village and just wants to spread chaos and destruction everywhere it goes, seeing all living things as mere playthings for its newfound power.
Another option for the main antagonist in 'naruto crimson avatar fanfiction' could be a spirit from another dimension. This spirit is attracted to the Crimson Avatar power and wants to use it to break through into the Naruto world completely. It starts by possessing weak - willed individuals and slowly builds up its strength, all while working towards its ultimate goal of taking over the entire world and bending it to its own alien will.
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.
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 an evil ninja who has also discovered the secret of the Crimson Saiyan God but wants to use it for destruction. He might be jealous of Naruto and see him as an obstacle to his plan of ruling the ninja world with this new power.
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.
Maybe it's a corrupted spirit from the avatar world that has become jealous of Naruto's unique powers as the Sage of Six Paths. This spirit could try to possess Naruto or those close to him in order to gain access to his powers and use them to rule over both the Naruto and avatar universes. It would be a very powerful and menacing antagonist as it can manipulate the elements in the avatar world and has knowledge of Naruto's world through its research on him.
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.
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.