Pain would be one of the most challenging. His Rinnegan gives him a wide range of powerful abilities like the Six Paths of Pain. Naruto would have to figure out how to counter each of those paths and find a way to reach the real Pain. Itachi is also tough. His Sharingan and genjutsu skills are top - notch. Naruto would need to break free from his genjutsu and then find a way to fight back against his fire - based attacks.
Naruto would become more confident. After destroying Akatsuki, he'd realize his true potential. He'd know he can take on the toughest enemies and protect his village. This new - found confidence would also make him more respected among the other ninjas.
One possible plot could be Naruto using his new Sage of Six Paths powers to take on the Akatsuki one by one. He could start with Hidan and Kakuzu, quickly dispatching them with his enhanced speed and strength. Then, he moves on to Deidara, outmaneuvering his explosive attacks with his own agility. Another plot might involve Naruto infiltrating the Akatsuki's hideout alone, using his shadow clones as decoys. He could then surprise Pain and defeat him in an epic battle, showing his growth as a ninja.
Yes, there might be such fanfics out there. In the Naruto fandom, with a large number of fans creating various types of stories, it's possible that some would write a story where Naruto destroys Sasuke in a way that could be seen as Sasuke - bashing. However, these kinds of fanfics may not be in line with the overall spirit of the original work which focuses on their complex relationship and growth.
Sasuke might be able to stop Naruto. They have a long - standing relationship and Sasuke has always been Naruto's equal in terms of power. He could use his Sharingan to try and break whatever is controlling Naruto or just fight him head - on and appeal to their old friendship.
The Akatsuki members themselves would have mixed reactions. Some, like Itachi (if he were still alive in the fanfic), might see Naruto as a potential asset but also a threat to their long - term goals. Kisame might be amused by Naruto's presence and see him as a new source of entertainment. Meanwhile, the Leaf Village elders would be outraged. They would see Naruto as a traitor and start devising plans to either bring him back or eliminate him.
It could be an organization similar to the Akatsuki, but with a different goal. Instead of collecting tailed beasts, they are focused on conquering villages. Their army would be made up of powerful and diverse fighters, and Naruto would have to face their leader, who has some sort of special power or ability that makes them a formidable foe.
The theme of justice might be present as well. If 'kina' is something evil or causing harm in the fanfic, Naruto's destruction of it can be seen as an act of justice. Also, the fanfic could explore the theme of sacrifice. Naruto might have to sacrifice something, like his time, energy or even relationships to be able to destroy 'kina' successfully.
It could be that Naruto is possessed by an evil spirit. This spirit wants to destroy the Leaf Village for its own reasons. Naruto, under its control, would first cut off the village's supply lines, making it difficult for the villagers to get food and resources. Then, he'd start attacking the ninja in the village one by one. He might even use his Nine - Tails chakra in a very destructive way, not caring about the harm it would cause to the place he once called home.