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.
The interactions would be quite diverse. Thor might see Saitama as a strange but powerful being. He could offer to have a friendly sparring match, which Saitama might decline because it's too much trouble. Black Widow would likely be trying to figure out Saitama's background and what makes him tick. And Spider - Man would be super excited to meet someone with such incredible power, constantly asking Saitama questions about how he got so strong.
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.
When it comes to Genos' abilities, the Avengers would be intrigued. Genos' ability to fire intense energy blasts could be a great asset in battles. Hawkeye might be thinking about how Genos could provide cover fire or take out tough enemies from a distance. But the Avengers would also be a bit worried about Genos' lack of emotional control at times. In his pursuit of power and revenge, he can be a bit reckless. The Avengers would need to work with him to make sure he doesn't go overboard. Also, Genos' loyalty to Saitama would be something they would need to understand, as it might influence his decisions within the team.
If Saitama joined the Avengers, it would be a game - changer. His incredible strength could easily defeat most of the villains they face. For example, Thanos might not even be a challenge for him. Saitama's simple - minded but determined personality would also bring a new dynamic to the team. He might not fully understand all the complex superhero politics, but he would just focus on getting the job done, which could inspire the other Avengers to be more straightforward in their approach.
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.
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.