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.
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 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.
In some fanfictions, Garou spares One Punch Man and then teams up with him temporarily to fight a greater evil. This unexpected alliance showcases their combined strength and also explores the dynamic between a self - proclaimed villain and an uninterested hero.
One idea could be exploring Garou's past more in - depth. Maybe show his childhood and what led him to become the way he is. Another could be a story where Garou has to team up with Saitama for some strange reason, like an even more powerful enemy emerging. Or perhaps a fanfiction where Garou tries to train under Saitama to become stronger in a different way than his usual self - taught methods.
Well, the fanfiction could focus on a training arc between them. Imagine Garou, who has his own unique fighting style, teaching Metal Bat some new moves. In return, Metal Bat could teach Garou about the importance of protecting the weak. Their relationship could develop from a mutual dislike to a begrudging respect and then to an actual friendship. This could lead to them fighting side - side against the various threats in the One Punch Man universe, and they might even change the public perception of Garou from a villain to an anti - hero.
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.