One possible plot could be that Fubuki, being frustrated with her lack of progress in terms of power compared to Saitama, decides to train with him. This would lead to a series of comical and at the same time, character - building moments. Saitama would probably give her some really simple but effective training advice that she initially scoffs at but later realizes is genius.
A plot for a fanfic could be that there is a mysterious force in the city that is nullifying all the heroes' powers except Saitama's. Fubuki is desperate as her powers are gone. Saitama offers to protect her and her group. During this time, Fubuki and Saitama have to rely on each other in ways they never expected. They might have to use their wits more as Saitama can't always just punch the problem away. This situation could bring them closer together and change their relationship dynamic.
One Punch Man is a really popular anime. Fubuki, as a character in it, has a lot of potential for fanfics. In a fanfic about them, it could explore their relationship in new ways. Maybe it could show Fubuki trying to recruit Saitama into her group in a more detailed and different way than in the original story. It could also develop her character further by showing her reactions to Saitama's overpowering strength in unique scenarios.
The fanfiction about Fubuki and Tatsumaki in One Punch Man is alluring because it gives a chance to expand on their characters. Fubuki, with her ambition to be a great hero, can be explored further. Maybe in a fanfic, she overcomes her insecurities related to her sister Tatsumaki's much stronger powers.
A possible plot is that the OC is a former enemy of Fubuki who has seen the error of their ways. They try to make amends by helping Fubuki in her quest to become stronger and more respected in the hero community. This could involve training sessions where the OC shares some unorthodox fighting techniques that complement Fubuki's psychic powers. The OC could also have connections to some powerful and mysterious figures in the One Punch Man universe, which could open up new storylines for Fubuki.
Many One Punch Man fanfics with Fubuki and Tatsumaki focus on their interactions with other characters too. They could be paired with Saitama in some comical or action - packed situations. Like, Fubuki trying to recruit Saitama into her group while Tatsumaki is constantly skeptical of his abilities.
As of the current storyline in the One Punch Man web comic, Fubuki is still alive. The plot doesn't involve her death.
One possible plot could be that Naruto is somehow transported to the One Punch Man universe. He might initially be confused by the different world but then start to adapt. He could team up with Saitama to fight against some powerful monsters. Along the way, Naruto tries to understand Saitama's seemingly effortless power while also showing Saitama the value of his own ninja way.
The OC can be introduced as a new recruit in Fubuki's group. For example, they could be a young and inexperienced hero who is in awe of Fubuki's powers and leadership. They start by following Fubuki's orders and gradually prove themselves useful in battles.
The plot could revolve around a villain who targets Saitama's hair specifically. This villain might believe that Saitama's power is directly related to his hair and tries to steal it or cut it off. Saitama then has to protect his hair while also dealing with the normal chaos of fighting monsters in the city.
A plot could involve Saitama ending up in the Naruto world. He quickly realizes that the power levels here are different. He may get involved in the ninja wars, using his one - punch ability to end battles quickly. Naruto, intrigued by Saitama's power, tries to figure out how he can achieve such strength. Meanwhile, the villains in the Naruto world start to fear this new, extremely powerful intruder.
It could be that the SI becomes a hero not in the traditional sense. Instead of having super strength or amazing powers like the other heroes, they use their wits. They might start a support group for heroes, helping them deal with the psychological stress of constantly fighting monsters. This would be a unique take in the One Punch Man fanfiction world, as most heroes are focused on physical combat. The SI could interact with heroes like Mumen Rider, who might be more receptive to this kind of support, and they could together change the way the hero community functions.