Another plot could involve Saitama using his new S - Class status to reform the hero system. He might see all the flaws in it, like how some heroes are more concerned with fame than actually saving people. So, he starts making changes from the inside. He could introduce new training methods or even a new ranking system that focuses more on a hero's selfless deeds rather than just their power level.
A possible plot is that Saitama gets involved in some sort of inter - hero politics as an S - Class hero. There could be rivalries between different S - Class heroes, and Saitama accidentally finds himself in the middle of it. Maybe one group wants to use his power for their own gain, while another wants to protect him from being exploited. Saitama, being Saitama, is just confused by all the drama and wishes he could just go back to his normal life of fighting monsters casually.
One possible plot could be that in this fanfiction, an alternate dimension where Saitama is a cannibal. Maybe there's a virus that has infected him and turned his moral compass upside down. Writers might explore how the other heroes react to this new, terrifying Saitama. Do they try to cure him? Or do they see him as a threat and try to stop him by force?
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.
A plot where Saitama travels to different dimensions in the One Punch Man universe. He could encounter strange versions of himself or other heroes and villains, leading to humorous and action - packed situations.
Genos would probably be even more in awe of Saitama. He already idolizes him, but with Saitama being S - Class, Genos might see him as even more untouchable. He could become more protective of Saitama's new status, trying to fend off any detractors or those who might try to take advantage of Saitama.
One interesting fanfiction could be where the S class heroes team up in a different way than in the original to face a new super - strong villain. Maybe they have to rely more on strategy rather than just brute strength, which gives a whole new perspective on their characters.
One interesting fanfiction storyline could be that Genos discovers a prophecy that Saitama is going to lose his powers. Genos then goes on a mad quest to find a way to prevent this from happening. He visits all kinds of strange places and consults with various powerful beings. Another could be that there is a competition among heroes to see who can train the best new recruit, and Saitama and Genos enter together. They face off against other hero - trainer pairs in various challenges. Also, a story where Genos creates a virtual reality world for Saitama to experience different kinds of battles that he normally wouldn't encounter in the real world, just for Saitama's amusement.
One possible storyline could be Boros surviving his encounter with Saitama and then seeking redemption. He realizes the vastness of Saitama's power and decides to train under him. This would be an interesting twist as Boros was initially a villain. Another could be a what - if scenario where Saitama and Boros team up to face an even greater threat that arrives on Earth. It would show their dynamic as they work together, with Saitama's nonchalant attitude and Boros' more serious and battle - hungry nature.
One creative plot could be a self - insert character who has the power to mimic Saitama's strength but only for a limited time. They use this power to enter a martial arts tournament in the One Punch Man universe. Another could be a self - insert that becomes Saitama's sidekick, trying to learn his secrets while also dealing with the comical chaos that follows Saitama everywhere. A third idea is a self - insert who is from a different dimension and has to prove their worth to Saitama by helping him with a particularly tough monster invasion in City Z.
One idea could be that Evil Saitama is actually a clone created by a mad scientist. The clone has all of Saitama's power but none of his morality. He starts to create chaos in the hero world, challenging heroes and causing destruction. Another plot could be that Evil Saitama is an alternate version from a parallel universe where he turned bad due to a tragic event in his past. He then travels to the main One Punch Man universe to seek revenge on the heroes there.
A cool plot might be a time - travel fanfiction. Saitama travels back in time to when he was just starting to become strong. He meets his past self and tries to guide him, but also has to deal with monsters from that era who are now different threats due to the time shift. And there could be a mystery element where Saitama has to figure out why time is acting strangely in the first place.