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.
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.
Well, a plot could be about an OC who is a healer. In the world of One Punch Man, where destruction is common, Tatsumaki might initially be dismissive of this healer OC. But as they face tougher enemies, Tatsumaki gets injured and realizes the value of the OC's power. It could be a story of her learning to work with someone whose power seems so different from her own. Another could be an OC who is a time - traveler. The OC comes to Tatsumaki's time to prevent a future disaster that involves her. Tatsumaki, being the proud and powerful character she is, is skeptical at first but gradually realizes the truth of the OC's claims. And for a third plot, an OC who can communicate with monsters. This OC tries to broker peace between Tatsumaki and the monsters, which Tatsumaki finds absurd at first but then starts to consider as the situation unfolds.
One popular plot idea is the gamer protagonist using in - game knowledge to strategize against One Punch Man villains. For example, they might use leveling up systems from games to enhance their own powers. Another is the gamer character getting involved in Saitama's daily life, like challenging him to a game - related task. And some fanfictions also have the gamer creating their own hero guild within the One Punch Man universe, using unique game - inspired recruitment methods.
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.
In a One Punch Man Blizzard fanfiction, a great plot could be that Blizzard is challenged by a rival group who claim to be stronger. Saitama, in his boredom, decides to help Blizzard prove them wrong. He does this in the most nonchalant way possible, like just one - punching all the opponents while eating an ice - cream. This makes Blizzard both frustrated and in awe of him. Another idea is that Blizzard discovers a hidden power within herself after being inspired by Saitama's confidence in his own strength.