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.
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.
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.