Even the villains would be affected. Saitama was a force that kept them in check, in a way. Without him, there would be a power vacuum. Some villains might see it as an opportunity to go on a rampage, but others might actually feel a sense of loss. Saitama was such a unique presence in the One Punch Man world that his death would send ripples through the entire ecosystem of the story.
The other heroes in the Hero Association would be in shock. Saitama was like an unbeatable force among them. His death would make them realize how vulnerable they really are. They would have to reevaluate their own strength and the safety of the world. Some might be inspired to train harder, while others could be paralyzed by fear.
Saitama would be shocked at first. He's used to Genos being around, always eager to learn from him. He might stand there in disbelief for a while.
One way it could be imagined is if a new, extremely powerful villain emerged. This villain might have the ability to nullify Saitama's strength, perhaps by some sort of reality - warping power. In the fanfiction, Saitama could be caught off - guard by this new threat, and despite his best efforts, succumb to the villain's attacks. It would be a very dark turn for the One Punch Man universe.
For Ruby, it would be a huge blow. She's the team leader and Weiss is her friend. Ruby would likely feel guilty, thinking she could have done more to protect her. Yang would be angry, both at the situation and potentially at herself. She might become more impulsive in her actions, seeking revenge. Blake would be deeply saddened. She might withdraw into herself, blaming her own past actions or inactions for Weiss's death.
The other characters would be deeply affected. Neville, for instance, would see Harry's sacrifice as an example of ultimate courage. He might be inspired to step up more in the fight against evil. Luna would be sad but also understand Harry's choice on a deeper level. She might see it as part of a greater cosmic plan. The teachers at Hogwarts, like McGonagall, would be heartbroken. Harry was like a son to many of them. They would feel a sense of pride in his selfless act but also a great sorrow. Hermione would be in a state of shock and disbelief. She would constantly replay the moment in her mind, wondering if there was anything she could have done to prevent it. This would change her personality, making her more withdrawn at first as she tries to come to terms with the loss.
The villains would have a mixed reaction. Some of the more power - hungry ones might see this as an opportunity to join forces with Saitama and cause even more chaos. However, there would also be villains who are afraid of Saitama's power, even if he's on their side now. They would be worried that he could turn on them at any moment. Overall, Saitama turning evil would completely disrupt the existing power dynamics in the One Punch Man world.
Tornado of Terror would likely be one of the first to try and take on Evil Saitama. She's powerful and confident, but she would soon realize the extent of his power. Mumen Rider, despite being weak, would still stand up to Evil Saitama out of a sense of justice. He would be quickly defeated but his act of courage might inspire other heroes to step up.
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.
The Hogwarts students would be greatly affected. The younger students who saw Harry as a hero would be in shock. They would lose their sense of safety and hope. The older students who knew Harry personally would feel a personal loss. The teachers, like McGonagall, would be grief - stricken. She had watched Harry grow up at Hogwarts and had seen his potential. His death would make her question the future of Hogwarts and the wizarding world. Even the ghosts at Hogwarts would feel a strange melancholy. Harry had interacted with them, and his death would be a significant event in the history of the school.
For Gibbs, it would be a huge loss. He has mentored Tony and they have a father - son - like relationship. Gibbs would likely become more withdrawn and even more obsessed with solving cases, as a way to distract himself from the pain. Ziva would feel a deep sense of guilt, as she and Tony have a complex relationship. She might blame herself for not being there for him when he needed her.
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.