One possible plot could be that Naruto is exposed to a powerful and dark jutsu that slowly turns him into a demon. As he transforms, he has to struggle with his new nature. His friends initially fear him but then try to find a way to reverse the process.
If Naruto becomes the Demon of the Mist in fanfiction, he might start to lose his sense of self. The title 'Demon of the Mist' implies a certain level of malevolence and mystery that is quite different from Naruto's usual sunny disposition. His ninja way would be severely tested as he tries to balance the new, darker identity with his old values. Maybe he'd be forced to fight against his former allies who see him as a threat now. Also, his appearance could change to something more in line with the image of a demon, which would be a shock to those who know him well.
Maybe there is an ancient wolf demon spirit that is corrupted and sees Naruto as a threat to its reign. This spirit would try to either kill Naruto or corrupt him further, causing chaos in the world.
One interesting plotline could be Naruto struggling to control his new wolf demon powers. He might accidentally lash out at his friends, creating a rift between them until he learns to master his abilities.
His relationship with Sasuke would become very strained. Sasuke, who has always been somewhat distrustful, would see Naruto as a real threat now. He might even consider Naruto an enemy and try to stop him.
His relationship with Sasuke would be severely strained. Sasuke, who has his own goals and views on power, would see Naruto's demon transformation as either a threat or a sign of weakness. He might distance himself from Naruto or even see him as an enemy to be defeated.
Maybe Naruto is cursed by an enemy. As a fox demon, his appearance would change drastically. His hair could become more like a fox's fur, and he might grow a tail or two. His personality could also be affected. He might become more mischievous like a fox, but still retain his core values of loyalty and friendship. He would then have to find a way to break the curse while dealing with the changes in himself.
His bond with Sasuke would take a new turn. Sasuke might see Naruto as an even greater rival now. He might be envious of Naruto's new powers on one hand, but also want to help him master them on the other. As for Sakura, she would be worried about Naruto. She would try to find medical jutsu to help him if there are any negative side - effects from becoming a fox demon.
If Naruto becomes a demon in a fanfic, it could lead to a whole new set of adventures. He might be shunned by the village he once protected. However, this could also be an opportunity for him to discover a new side of his power. He could use his demonic powers for good, but first he would need to overcome the prejudice against him. His relationships with characters like Sasuke and Sakura would be put to the test as they might not trust him immediately.
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.