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.
Maybe Tatsumaki would be impressed by the power display or decide to interfere to show her own strength. It all depends on how the fanfic writer imagines her reaction and actions.
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.
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.