One possibility is that Naruto could be betrayed by the village he loves so deeply. Maybe false accusations of being a traitor are made against him. This heartbreak and sense of injustice could be the catalyst for his turn to evil. He might then use his powerful jutsu against Konoha, believing that they don't deserve his protection anymore.
The Hidden Leaf Village itself could become the main antagonist. Since Naruto turns evil, the village that once nurtured him would now be against him. The village elders and some of the ninja who oppose Naruto's new evil ways would be in conflict with him. For example, Kakashi might be forced to fight against Naruto to protect the village.
Another example could be a 'pure evil Naruto fanfic' where Naruto is written as having a completely distorted view of the world. He might believe that power is the only thing that matters and starts to hunt down strong ninjas just to prove his dominance, regardless of whether they are good or bad. He could use underhanded tactics like attacking from the shadows or using innocent people as shields.
Regular Naruto fanfic usually focuses on positive aspects like Naruto's growth as a hero, his friendships, and his journey to protect the village. But 'pure evil Naruto fanfic' turns all that on its head. It makes Naruto the antagonist instead of the protagonist.
One possibility could be that Naruto is drawn to boxing because of its physicality and the determination it requires. He might see it as a new way to test his strength and endurance outside of ninja battles. Maybe he starts training with a local boxing coach who sees his potential and helps him develop his skills.
Perhaps Naruto's transformation into a Predator - like being changes his appearance. He might have some Predator - like physical features such as glowing eyes or enhanced strength. This could lead to his friends initially being scared or confused, but as they see him use these new powers for good, they come to accept him. In the story, he could also learn to control these powers, so they don't overtake his original ninja nature.
One possible scenario in a fanfiction where Naruto becomes Pein could be that Naruto is exposed to some dark jutsu that alters his mind. This jutsu might be related to the Rinnegan's power. As a result, he starts to adopt Pein's ideology of bringing pain to the world in order to create peace. His friends would be shocked and try to bring him back to his normal self.
Well, Naruto as a genie would be a really cool idea for a fanfiction. His character already has a strong sense of justice and a big heart. As a genie, he would be able to spread his positive influence even more widely. Imagine Naruto granting the wish of a poor village for food and resources. He could also use his genie powers to create illusions on a grand scale, fooling the enemies of the village. And his bottle or lamp (where he is trapped as a genie) could be hidden in some super - secret location in the Hidden Leaf.
One possible scenario could be that Naruto is transported to the Flash's universe and somehow gains the powers of Savitar. He might use Savitar's speed in his own unique way, like combining it with his shadow clones to create a sort of blitzkrieg attack. Maybe he would also struggle with the darker aspects of Savitar's power, as Naruto is typically a more positive - minded character.