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.
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.
One great 'Garou My Hero Academia fanfiction' is 'Garou's Redemption'. It explores Garou's character in a different light, showing his journey towards understanding the true meaning of heroism. Another is 'Garou Among the Heroes' which places Garou in various situations with the heroes, creating interesting interactions.
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.