Perhaps in this fanfiction, the Akatsuki from Naruto are hired by an evil organization in the One Punch Man world. Naruto and Saitama have to track them down. Along the way, Naruto is amazed by Saitama's simple yet overpowered strength, while Saitama is intrigued by Naruto's ninja skills and never - give - up attitude. They combine their unique fighting styles to take on the Akatsuki members and the new villains.
One idea could be that Naruto somehow gets transported to the One Punch Man universe. He teams up with Saitama to fight a common enemy that has the ability to absorb chakra and is also super strong physically. They start off not understanding each other's powers but gradually learn to work together.
A great fanfiction idea could involve the ninjas of Naruto trying to figure out how Saitama got his incredible strength. They might study him in the hopes of creating their own super - soldiers. Another idea is a time - travel fanfic where Saitama goes back in time to the founding of the ninja villages in Naruto. And there could be a story where the One Punch Man characters get chakra and start using ninja techniques in their own battles.
One idea could be that Mob and Saitama accidentally switch powers for a day. Mob would be super strong but struggle with his usual self - control, while Saitama would have to deal with Mob's psychic powers which he has no experience with.
One idea could be that Saitama enrolls in the school from 'Class'. He'd be this super powerful student that no one really understands at first. His nonchalant attitude towards the school's challenges would be really funny.
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.