A plot could involve a villain in the DoS (Darkness or Shadow) realm who has the ability to nullify Saitama's strength for short periods. This villain is causing chaos in the city, and Saitama has to figure out how to defeat him without relying solely on his brute force. Maybe he teams up with other heroes who have skills in detecting and countering such abilities.
Definitely Saitama. He's the main character and his nonchalant attitude towards his incredible power is always interesting to explore in fanfiction. Also, Genos. His loyalty to Saitama and his own journey to become stronger make him a great addition.
First, think about their power. In One Punch Man, powers can be really wacky. So for your OC, it could be something like the ability to turn into any type of food, which would be both funny and unique. Then, consider their backstory. Maybe they were an ordinary chef who got their powers from a strange experiment in the kitchen. Also, don't forget about their personality. They could be really laid - back and love to share their food - power with others. This way, your OC will fit well into the One Punch Man DoS fanfiction world.
One interesting storyline could be that your OC is from another dimension and has come to the One Punch Man world. They bring with them a different set of powers and values. This could lead to all sorts of clashes and also new alliances. Maybe they initially think the heroes in this world are too laid - back or too violent, and they try to change the way things are done. Or perhaps their power is something that is desperately needed in this world to fight a new and powerful enemy that the regular heroes can't handle on their own.
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.