A plot could involve an experiment gone wrong in the One Punch Man world that creates a rift, pulling in inklings and octolings. Genos and Saitama have to deal with these new, ink - based beings. The inklings mistake Saitama for a super - strong boss - like character and try to challenge him. Saitama, of course, defeats them with one punch, but then realizes they're not really bad. So he teams up with them to stop the real threat, which is a mutated sea creature that has been affected by the same experiment and is now attacking both worlds.
In a crossover, Saitama might be the odd one out in the colorful and ink - filled Splatoon world. But his simplicity and straightforwardness would attract the inklings. They would try to get him to embrace their culture, like trying different ink colors. Saitama would just go with the flow. Genos, on the other hand, would be more focused on how the Splatoon world could help him become stronger. He would observe the inklings' and octolings' teamwork during battles. The inklings and octolings would be fascinated by Genos' loyalty to Saitama and might try to understand the concept of a master - apprentice relationship in the context of their own social structures.
Another idea is that Naruto, curious about other powerful beings, uses a jutsu to travel to the One Punch Man universe. There, he would be amazed by Saitama's strength. Naruto could try to learn from Saitama how to be so effortlessly strong, but Saitama would probably just shrug it off and say there's no special trick. They could team up to fight some of the monsters in Saitama's world, with Naruto using his ninjutsu and Saitama just punching things away.
A fanfiction might center around a competition between Popeye and Saitama. They could have a friendly contest to see who can defeat the most villains in a day. Popeye would use his wits and spinach - enhanced strength, while Saitama would rely on his incredible punching power. This could lead to some hilarious and exciting encounters as they both try to outdo each other.
One common theme is the exploration of Saitama's power in different universes. For example, when he crosses over to a universe with magic, how his physical strength compares to magical abilities.
A crossover with 'Dragon Ball' would be cool. Picture Saitama facing off against Goku. It would be an interesting contrast between Goku's love for a good fight and Saitama's boredom with opponents who can't challenge him.
One possible plot could be that Saitama somehow ends up in the Marvel universe. He might be mistaken for a new superpowered being by the Avengers. Maybe he just casually punches a big bad like Thanos during one of his attacks, completely oblivious to the whole situation, leaving the Avengers both shocked and amused.
How about a crossover with 'Attack on Titan'? Saitama could find himself in the world of the Titans. His ability to defeat any enemy with one punch would be extremely useful against the Titans. He might not understand the complex politics and fear that surrounds the people in that world at first. But as he starts taking down Titans left and right, he would become a sort of legend among the survivors.
The plot could involve Garou somehow ending up in Saitama's world more permanently. Maybe he decides to team up with Saitama against a common enemy. This new enemy could be a powerful being from another dimension that neither of them could face alone. Their contrasting fighting styles would make for an interesting dynamic, with Saitama's one - punch power and Garou's martial arts skills.
It's hard to say. Saitama has the ability to defeat any enemy with one punch, but Goku has an array of powerful forms and techniques. However, Saitama's strength seems to have no limit that we've seen so far. So, in a straight - up fight, Saitama might have the edge just because of his one - punch ability.