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.
The OC could be a healer type character in the fanfiction. Fubuki often gets into dangerous battles, and the OC's presence could be crucial for her survival. The OC might not be a very strong fighter on their own but has the ability to quickly heal Fubuki's wounds and restore her psychic energy. This would lead to a relationship where Fubuki has to protect the OC in battle while relying on them for support. Their adventures together could revolve around finding rare items or places that can enhance the OC's healing abilities even further.
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.
Here are some plot ideas. The Dragon OC could be a reincarnation of an ancient and powerful dragon. It gradually remembers its past life and powers as it interacts with the characters in One Punch Man. It might start as a weak - looking creature but as it unlocks its memories, it becomes extremely powerful. Another plot could involve the Dragon OC being hunted by a secret organization in the One Punch Man world because of its unique dragon DNA. This organization wants to use its DNA to create super - soldiers, and the Dragon OC has to fight back and expose this evil plan.
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.
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.
One plot could be Tornado having to team up with a new, unexpected hero to face a powerful villain that even she has trouble with. This would show a different side of her as she has to work with someone she might not initially get along with.
One idea could be that Saitama from One Punch Man somehow ends up in the world of Overlord. He might be mistaken for a powerful player at first. Since he's so strong, he could easily defeat many of the powerful monsters in the Overlord world, which would cause a stir among the various guilds and NPCs. Another plot could involve the characters from Overlord trying to understand Saitama's power, which is beyond their comprehension as it's not based on magic or typical RPG stats.
A plot could involve a villain who has the power to control hair and targets Saitama because his hair is so unique. This leads to a battle where Saitama has to figure out how to fight without using his hair as a weapon, which is quite difficult since his every move usually involves his head (and thus his hair) in some way.
One idea could be Issei getting transported to the One Punch Man universe and trying to prove his worth among the powerful heroes there. He might start by challenging some of the lower - ranked heroes and learning the ropes of this new world.
Another plot could involve Sii being a civilian who accidentally gets caught in the middle of a fight between Saitama and a villain. Sii then discovers they have a latent psychic ability that they use to help Saitama in the fight, and this leads to Sii becoming an apprentice of sorts to Saitama, learning about being a hero.
In a One Punch Man fanfiction, a great plot could be the origin story of some of the minor heroes. For example, how Mumen Rider got his bike and started his hero journey. Another plot could be a tournament among the heroes to see who is the most skilled (not the strongest since that's Saitama). And a story where Saitama has to deal with a PR disaster for the Hero Association because of his non - traditional hero behavior.