Issei and Tatsumaki would also be a fascinating pair. Tatsumaki is a very powerful and prideful esper. Issei, with his own set of powers and a bit of a brash personality at times, could either clash with Tatsumaki or, in an unexpected turn, they could find common ground and work together against a common enemy.
Definitely Naruto and Saitama. Naruto's energetic and determined personality against Saitama's laid - back and seemingly bored attitude would create a great contrast. Naruto would be constantly amazed by Saitama's power, while Saitama would be a bit puzzled by Naruto's complex world of ninjutsu and chakra.
Ruby and Saitama would be an interesting pairing. Ruby is young and full of energy, always looking to improve, while Saitama is already overpowered but has a simple - minded approach. Their interactions could be both humorous and inspiring. Another great pairing could be Weiss and Genos. Weiss is elegant and proper, and Genos is a cyborg with a serious demeanor. They could contrast and complement each other well.
Frisk would also be a great character to focus on. If they were to gain some of Saitama's power, it would completely change their journey in the Underground. They could use their new strength to protect the monsters instead of just trying to find a way out. This would lead to new relationships and storylines. They might be able to stand up to Flowey in a different way, and even change the way the entire Underground views humans.
Saitama and Albedo would have really interesting interactions. Albedo is so devoted to Ainz in Overlord, and when she meets Saitama, who is just this nonchalant super - powerful guy, she would be both confused and perhaps a bit threatened. Saitama, on the other hand, would just see her as another person in this strange new world.
Definitely Saitama and Deadpool. Deadpool would be constantly trying to break the fourth wall and make jokes, while Saitama would just be there with his usual blank expression. It would be hilarious to see how Saitama reacts to Deadpool's non - stop chatter.
A really interesting character interaction in Garou fanfiction could be between Garou and Genos. Genos is so focused on becoming stronger and avenging his family, and Garou is on his own path of power - seeking. Their clash could be both physical and ideological. Genos might see Garou as a threat to the order he wants to protect, and Garou might view Genos as just another 'fake' hero.
Harry Potter himself would be an interesting character to crossover into the One Punch Man universe. He would be completely out of his element in a world full of superheroes and monsters. His magic skills would be a new and strange power there. He could team up with Saitama and learn about a different kind of heroism. Also, his experiences with fighting Voldemort could give him unique insights when dealing with the villains in One Punch Man's world.
I think Eren and Saitama would also be a great pair. Eren has his complex motives and the power of the Titans, and Saitama is so simple - minded yet incredibly strong. They could interact in a really unique way, like Eren being all fired up to fight Titans and Saitama just being like 'Oh, that's nice' and then one - punching all the Titans. It would be a contrast of personalities and powers.
Another interesting pairing could be Tatsumaki from One Punch Man and Erza from Fairy Tail. Tatsumaki has powerful psychic abilities and a somewhat haughty personality. Erza, on the other hand, is a strong and disciplined mage. They could have a rivalry at first, both being very confident in their own powers. But as they face a common enemy together, they could learn to respect each other's abilities and form an unlikely alliance.
Definitely Mob and Saitama. Their personalities are so different. Mob is a shy and emotional high - school boy with powerful psychic abilities, while Saitama is a bored and seemingly emotionless hero who can defeat any enemy with one punch. Their interactions would be full of comical misunderstandings and also opportunities for character growth.