Another option is a rival village that discovers the potential of the shrine maiden's powers. They send their best ninja to kidnap her and force her to use her powers against the Hidden Leaf. This antagonist would be more of a political and military threat, as they are using the shrine maiden as a tool to gain an advantage in the shinobi world.
Obviously, Naruto himself could be one of the main characters. He would interact with the shrine maiden, perhaps learning about her culture and powers. Another could be the shrine maiden, of course. She would be at the center of the story with her unique abilities and background.
Yes, there are likely fanfictions out there with this concept. Fanfiction platforms have a wide variety of creative takes on existing characters. You can try searching on popular fanfiction sites like Archive of Our Own or FanFiction.net. Just type in relevant keywords like 'female Naruto shrine maiden' and browse through the results.
Perhaps it's about how her being a shrine maiden changes her relationship with Sasuke or other characters. For instance, they might view her in a different light, with more respect for her spiritual role. It could also explore how she uses her shrine maiden powers, if any, in battles or in solving village problems. Also, it might show her learning the ways of the shrine maiden, like the proper ceremonies and the significance of the shrine she is associated with.
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.
It could be a rogue Kumo ninja who is jealous of Naruto's power and wants to prove himself by defeating Naruto. This ninja might have some dark secrets and a grudge against Kumo as well.
It could be Orochimaru. He's always been interested in immortality and reanimation techniques. Maybe he reanimates Naruto for some evil purpose, like using Naruto's power to take over the ninja world.