Kawaki makes his appearance in Boruto manga at a specific point, but I can't recall exactly. You might want to check the official manga sources for the precise chapter.
One common theme is redemption. Since Garou is often portrayed as an anti - hero or a villain, many fanfictions explore his path towards redemption. Another theme is power struggle. Given the power - filled world of My Hero Academia, fanfictions often deal with how Garou's power relates to that of the heroes and villains.
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.