In this crossover, the characters' dynamics would be quite interesting. Garou, known for his anti - hero stance, would be in a world full of heroes when he crosses over to One Punch Man's universe. His interactions with Saitama would be a clash of ideologies. Saitama represents the pure, uncomplicated power of a hero, while Garou is all about challenging the status quo. As they interact, Garou might start to see the value in Saitama's simplicity, and Saitama might start to understand Garou's deeper motives for his actions.
If we consider a crossover fanfiction, Saitama's dynamic with Boros would be quite different. Saitama might start to see Boros not just as an opponent but as someone with a complex backstory. Boros, on the other hand, would probably go from seeing Saitama as a mere target to someone he has to respect. Genos might become more cautious around Boros, seeing his power first - hand. And other heroes in the One Punch Man world would be more on edge with Boros around, changing the overall group dynamic.
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.
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.
If Saitama is a cannibal in the fanfiction, it completely changes his relationship with Genos. Genos, who is usually very loyal, might now be horrified and conflicted. He might not know whether to help Saitama or stop him.
Another plot could involve a crossover with a different anime universe through a strange portal. For example, if he entered the world of My Hero Academia. Garou could initially try to assert his dominance but quickly realizes that the quirks in that world are different from his martial arts skills. He might then start to learn about these new powers and how to adapt, all while causing chaos and also forming unexpected alliances.
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.
There could also be a fanfiction that delves into Garou's past more deeply. Maybe we learn about his family and how his upbringing led him to his current views on heroes and villains. By exploring his childhood traumas and experiences, we can better understand why he is so obsessed with becoming the ultimate monster and how those roots could be the key to his possible transformation in the story.
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.
The manga has more detailed storylines and character developments. It gives a deeper insight into Garou's character.
First, you should really understand Garou's character. Read the original manga or watch the anime to get a good feel for his personality, his motives, and his fighting style. Then, come up with a unique concept, like the ones I mentioned before. It could be a new adventure for him or a different take on his relationships. After that, just start writing. Don't worry too much about making it perfect at first.