Well, in such a fanfiction, Naruto as a monster could be a result of an experiment gone wrong in a crossover world. His transformation might make him an outcast even among his own village. He would have to struggle with his new identity. His monstrous form could attract other evil forces that want to use his power for their own malicious purposes, while Naruto, deep down, still has his good heart fighting against the monster within.
Naruto would likely experience a significant shift in his personality. He might become more aggressive and less trusting as he turns into a monster. His friends would also change in their approach to him. For instance, Shikamaru, known for his intelligence, would have to come up with new strategies to deal with Naruto's new state. He might be more cautious around Naruto.
The story could progress with Naruto being shunned by those he once called friends. He might then be on a journey to find a way to reverse his transformation. Along the way, he could encounter other characters from different universes who either help or hinder his quest.
It could be Orochimaru. He has always been interested in experimenting with new forms and powers. If Naruto becomes a monster, Orochimaru might see him as a perfect specimen to study or use for his own evil plans.
One possible plot could be that Naruto is experimented on by a rogue ninja group using forbidden jutsu, which turns him into a monster. As a monster, he struggles to control his new powers and is shunned by the village at first. But then he uses his new form to protect the village from a greater threat.
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.
Well, perhaps Percy could start off as a normal person who is suddenly intrigued by the world of monsters. Maybe he gets cursed and starts to transform into a monster himself bit by bit, and has to figure out how to reverse it while also dealing with his newfound monster - like instincts.
In such a fanfiction, Harry might discover new powers related to being a monster. For example, he could have enhanced strength or senses. Maybe he would struggle to fit in with his old friends at Hogwarts as they might be afraid of his new form.
If Saitama became a monster in fanfiction, his physical appearance would surely change drastically. His bald head, which is now a symbol of his simplicity and power, might take on a more menacing look. His eyes could turn red, and his muscles might bulge in an even more grotesque way. This transformation could also affect his relationships. Genos, for example, would be in a difficult position. He idolizes Saitama as his master, but if Saitama becomes a monster, Genos might have to fight him to protect the city.
One possible plot could be that Naruto's death in Wave shocks the ninja world. His body then undergoes some dark transformation due to a hidden curse or experiment. As a monster, he might lose his sense of self but retain his ninja skills. His former friends like Sasuke and Sakura would be torn between trying to save him or having to fight him to protect the village. The story could explore themes of loss, the power of darkness, and the struggle between friendship and duty.