Madara Uchiha could also be an antagonist. Even if he is dead in the original story, in fanfiction, he could be brought back in some way. Since he has a deep connection with the Uchiha and the Rinnegan, he might see Naruto's awakening as an abomination or a threat to his own legacy and try to eliminate Naruto.
A new, original character created specifically for the fanfiction could be an antagonist. This character could be from a long - lost clan that has a grudge against the Uzumaki or the Senju. When Naruto awakens the Rinnegan, they see it as a chance to take revenge on the main lineages that they believe wronged their clan in the past and target Naruto.
One potential antagonist could be Orochimaru. He is always after powerful jutsus and the power of the Rinnegan would be extremely appealing to him. He might try to either steal Naruto's newly awakened Rinnegan or prevent Naruto from fully mastering it so that he can be the most powerful one in the ninja world.
There could also be new characters introduced as antagonists. In this fanfiction, perhaps there are dark entities that are either drawn to Naruto's new evil self or are competing with him for control. They might have powers that rival or complement his Rinnegan - based abilities, leading to some intense battles and power struggles.
In some fanfictions, Naruto might awaken the Rinnegan through a powerful jutsu overload. For example, if he combines a super - charged Rasengan with some ancient and forbidden chakra - based techniques. This could potentially trigger a genetic or chakra - system shift that results in the awakening of the Rinnegan.
One potential antagonist could be a rogue ninja who also has aerokinesis but uses it for evil. He might be jealous of Naruto's natural talent and try to undermine him at every turn. Another could be a powerful sorcerer from a far - off land who has come to the Naruto universe specifically to find someone with aerokinesis to either steal their power or use them as a tool. There could also be a group of anti - ninja who see Naruto's new power as a threat to their plans of overthrowing the ninja system and they try to eliminate him.
In some fanfictions, Naruto might awaken his Rinnegan through a powerful and ancient jutsu. For example, he could come across a hidden scroll that contains the knowledge and power to unlock this dojutsu. Maybe a mysterious sage or an old shinobi reveals the secret to him, guiding him through a series of trials that eventually lead to the awakening of the Rinnegan.
Maybe a new organization of rogue ninjas that see Naruto's Rinnegan as a threat to their own goals of taking over the ninja world. They target Naruko as a way to get to Naruto. These ninjas could have some unique and powerful jutsus of their own, making them formidable antagonists.
The Akatsuki as a whole could be antagonists. In fanfictions, the Akatsuki might be interested in the power of Naruto's Evil Rinnegan. They could try to capture him to extract the power or use him in some way for their own goals. Pain, being a powerful member of the Akatsuki, could also be a specific antagonist. His own ideology about pain and peace might clash with Naruto's new, potentially more chaotic state with the Evil Rinnegan. Additionally, new original antagonists created specifically for the fanfiction could be common. These could be dark, powerful figures who are drawn to the power of the Evil Rinnegan and want to either possess it or destroy Naruto to prevent its misuse.
It might be related to a connection with some powerful entity in the Wave arc. For example, if there's a secret shinobi from the past whose power is tied to the Rinnegan and Naruto somehow forms a bond with that power source, it could lead to the awakening. Also, the intense emotions and challenges he faces in the Wave arc might be a catalyst, pushing his chakra to a new level and awakening the Rinnegan.
Perhaps Orochimaru could still be an antagonist. In this fanfiction, he might be jealous of Naruto's new god - like gaming powers and try to steal them for his own experiments.
There could be a jealous fellow ninja from within the village who feels that Naruto is getting too much attention for his blade - mastery. This ninja could turn against Naruto and become an antagonist, perhaps even teaming up with external forces. Also, an ancient evil entity that was sealed away long ago might be awakened by the power of Naruto's blade and seek to destroy him and the village to regain its freedom.
There could be new antagonists created specifically for this fanfiction. Maybe some other ninja from a far - off land who are jealous of Naruto's power. They could come to the Hidden Leaf Village to challenge Naruto and steal his speed - related secrets.