One possible plot could be that the Child Emperor, with his high intelligence and various inventions, is initially intrigued by Garou's self - proclaimed 'hero - hunting' ideology. They might have a series of battles where the Child Emperor uses his technological gadgets to counter Garou's martial arts skills. Eventually, they could find some common ground in their views on the nature of heroes and villains, leading to an unexpected alliance or at least a mutual understanding.
It's really hard to say. The Child Emperor has his amazing technology and intelligence on his side. He can create all sorts of gadgets to attack and defend. But Garou is a really tough fighter with great martial arts skills and the ability to quickly adapt in battle. If the fight is in an open area, Garou might be able to use his speed and agility to dodge the Child Emperor's attacks and get close enough to land a powerful blow. However, if the Child Emperor has time to set up his defenses and traps, he could potentially wear Garou down. So it could go either way depending on the circumstances of the fight.
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.