One idea could be a story where Garou and Metal Bat are forced to team up against a common enemy stronger than they've ever faced. They start off hating each other but gradually learn to respect each other's fighting styles.
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.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics from various fandoms, and there are likely to be quite a few 'garou x metal bat' ones there. Just use the search function and type in the relevant tags.
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.
One idea could be that they start off as rivals but are forced to team up against a common, even more powerful enemy. As they fight side by side, they gradually gain respect for each other's fighting abilities.
There is also 'Metal and Fang'. In this fanfic, it focuses on how Metal Bat and Garou start off as rivals but gradually come to understand each other's motives. It's quite interesting as it delves deep into their personalities and how they might interact in different situations.
There's a fanfic titled 'Garou and Metal Bat: A New Understanding'. In this story, they start off as rivals but as the plot progresses, they realize they have more in common than they thought. It shows their journey from hostility to a sort of friendship, with some great action scenes in between.
How about a plot twist where Garou suddenly loses his powers. Metal Bat, who has always seen Garou as a powerful adversary, now has to protect him. This forces them to work together in a new way and also makes Metal Bat re - evaluate his perception of Garou.
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.
In the fanfiction, put them in various challenging situations. For example, they are trapped in a maze - like place. Metal Bat has to rely on Garou's survival skills, and Garou has to rely on Metal Bat's brute strength at times. Through these experiences, they both change. Metal Bat becomes more patient and Garou becomes more open to the idea of not being a complete outcast. This way, their characters develop in a natural and engaging way.