Well, considering a fanfiction about Saitama meeting Tornado and how it affects the Hero Association. Their meeting could start a new era in the association. Saitama's unique power and Tornado's psychic abilities could inspire other heroes to train harder or explore different ways of using their powers. Their combined presence could also make the association more appealing to potential new heroes. The association might also start to focus more on the individual strengths of each hero, like Saitama's simplicity and Tornado's complexity, and try to find ways to blend these different styles for better overall performance.
In this fanfiction scenario of Saitama meeting Tornado, the story could be centered around their different ways of dealing with threats. Tornado is used to using her psychic powers in a more elaborate and showy way. Saitama, on the other hand, just punches. Their meeting could lead to Tornado being curious about Saitama's simple yet extremely effective method. Maybe she would try to get him to teach her his way, but Saitama would be like 'There's nothing to teach, I just punch'. This could lead to some humorous exchanges between them as they interact more.
In a fanfiction scenario between Tornado and Saitama, Saitama would likely win. He's so overpowered that he can defeat any opponent with one punch. Tornado is extremely strong with her psychic powers, but Saitama's strength seems limitless. His simple training routine has made him so powerful that no enemy has been able to withstand his punch so far.
One possible plot could be that Tornado initially challenges Saitama out of pride. She sees his nonchalant attitude and wants to prove that her psychic powers are superior. Saitama, being Saitama, just goes along with it. As the battle unfolds, Tornado throws everything she has at Saitama, but he just dodges casually. Eventually, Saitama throws a light punch that knocks Tornado back, but not seriously hurt, which makes her realize his true strength.
Saitama might be really frustrated with Mosquito Girl. Since he can defeat most powerful foes with one punch, but a mosquito girl would be a different kind of annoyance. She could be constantly buzzing around and trying to bite him, and Saitama would probably just try to swat her away like a normal mosquito at first. But when he realizes she's a 'villain' in her own right, he might just give her one quick punch to end the nuisance.
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.
It makes Saitama an even more overpowered character, adding to the comical and extreme nature of the story.
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.
For a good character interaction between Garou and Saitama, start with their fundamental differences. Saitama is a hero who is bored with the lack of challenge, while Garou hates heroes. Have them start with a fight, but make it a comical one where Saitama's nonchalance annoys Garou to no end. This can lead to some great dialogue exchanges where Garou tries to taunt Saitama into taking him seriously, but Saitama just doesn't care.
One interesting storyline could be about a new hero rising through the ranks of the Hero Association in an unexpected way. Maybe a hero with no super strength but extremely high intelligence devises unique strategies to defeat monsters, which challenges the traditional view of what a hero should be in the One Punch Man universe.
In a fanfiction, Mosquito Girl could develop a grudging respect for Saitama. After being defeated by him, she realizes that there are beings far more powerful than her. And Saitama, on the other hand, might start to see Mosquito Girl as more than just a villain. Maybe he discovers that she has some good qualities, like her tenacity. So their relationship could shift from pure enmity to a more complex dynamic of mutual understanding.