It could be a powerful priest or someone who represents the traditional view of kami. They might see Naruto's forsaking of kami as a threat to their power and beliefs, and thus oppose him.
Maybe there's a group of fanatics who believe in the absolute authority of kami. When Naruto forsakes kami, they become his enemies. They could try to stop him from spreading his new ideas or from finding an alternative way that doesn't involve kami.
In this 'naruto forsakes kami fanfiction', it could be that Naruto has seen the destruction and harm caused by the dogmatic following of kami. He realizes that the strict rules and expectations associated with kami are holding him back or causing harm to others. So, he decides to break away from it and create his own path, which might involve exploring different forms of power or a new moral code.
Obviously, Kami would be the main antagonist. Since the title says 'naruto betrayed by kami', Kami is the one doing the betraying, which makes him the bad guy in this fanfiction.
Maybe a rogue ninja who has made a pact with a dark force. This ninja could see Naruto as a threat to his plans of ruling the ninja world, especially with his new status as a dragon warrior of the kami. He would try to stop Naruto at all costs and might have some dark powers of his own.
Maybe a rogue ninja who has somehow tapped into dark powers related to Kami's domain. This ninja wants to use these powers to take over the ninja world, and Naruto, with Kami's help, has to stop him. In this fanfiction, the rogue ninja could have some personal vendetta against Kami as well, which adds more depth to the story.
Maybe it's a completely new and unknown kami. A kami created specifically for this fanfiction. This kami could have unique powers and a different view of the ninja world compared to the existing characters in Naruto. It would add a fresh and exciting element to the story.
Maybe it's an existing character from Naruto who has been elevated to Kami status. For example, Tsunade could be turned into a Kami, and Naruto's relationship with her could be a very different dynamic from their previous ninja - leader relationship. Or it could be Kaguya, who was already a very powerful being, and in this fanfiction, she reforms and becomes a Kami and Naruto's wife.
In fanfiction, the 'kami' could be an original deity created by the author. This new 'kami' could have its own motives and reasons for giving a gift to Naruto. Maybe it's a kami that has been observing Naruto's journey and decides to interfere to help him achieve a greater purpose. It could be a kami associated with nature, like a kami of the wind or the fire, and the gift could be related to that element.
Sasuke could also be a great main character for this kind of fanfiction. With his already complex character arc in the original series, adding the kami bloodline to his story could take it in new and exciting directions. He could struggle with the power of the bloodline, perhaps fearing it would make him too much like those he has fought against in the past, or he could use it to seek revenge in a more powerful way than before.
Perhaps it could be a female version of an existing kami from Naruto lore. For instance, a female counterpart of the Sage of Six Paths. She would have vast knowledge and power, and her training of Naruto would be a way to pass on that ancient wisdom and strength.
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.