For a good character interaction between Garou and Saitama, start with their fundamental differences. Saitama is a hero who is bored with the lack of challenge, while Garou hates heroes. Have them start with a fight, but make it a comical one where Saitama's nonchalance annoys Garou to no end. This can lead to some great dialogue exchanges where Garou tries to taunt Saitama into taking him seriously, but Saitama just doesn't care.
Well, in the fanfiction, the moment when Cosmic Garou tries to counter Saitama's attacks with his own new - found cosmic powers is really exciting. He has evolved to a whole new level, and seeing how his powers match up against Saitama's is a big part of the thrill. Maybe he creates some sort of cosmic energy blast that Saitama has to dodge or block in an unexpected way.
Saitama is a very powerful hero in the One Punch Man universe. He can defeat any opponent with just one punch. He has a simple appearance and a somewhat laid - back personality.
The battle between Saitama and Cosmic Garou in the fanfiction would have a catastrophic effect on the environment. First, Saitama's initial attacks might send out concussive forces that uproot trees and scatter them like matchsticks. Cosmic Garou's response could involve creating some sort of cosmic vortex that sucks in everything around it, from small pebbles to large boulders. As the fight progresses, the area around them would become a desolate wasteland, with fires raging from the heat generated by their attacks and the air filled with dust and debris, making it difficult for any living thing to survive in the vicinity.
Character development is a big part of these fanfics. Since both characters have their own fan bases, the writers try to stay true to their original personalities while also adding new layers. For example, Garou's struggle between his anti - hero ways and his potential for good can be further explored in relation to Metal Bat's more straightforward sense of justice. And the action scenes in these fanfics are usually intense, given the combat - ready nature of both Metal Bat and Garou.
Some smaller, more niche fanfic communities or forums dedicated to the source material of Suiryu and Garou might also be a good place to find such fanfics. You can ask around in those communities for recommendations or search through their story archives.
A fanfic could explore a world where Suiryu and Garou are from different dimensions. They meet by chance and have to figure out how to get back to their respective worlds while also dealing with their own personal growth and the challenges that come with their encounter.