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 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.
Well, in the fanfiction, the moment when Cosmic Garou tries to counter Saitama's attacks with his own new - found cosmic powers is really exciting. He has evolved to a whole new level, and seeing how his powers match up against Saitama's is a big part of the thrill. Maybe he creates some sort of cosmic energy blast that Saitama has to dodge or block in an unexpected way.
It depends on the context. If it's a battle of pure physical strength and chakra, Naruto might have the edge as he has years of experience and has faced more powerful foes. But Boruto has some unique abilities like his Jougan. So if the fight involves using new - age ninja techniques, Boruto could surprise Naruto.
It could also be related to a prophecy or a pre - determined fate in the fanfic's plot. There might be some kind of dark prophecy that says Kawaki has to kill Naruto to prevent a greater evil. Or perhaps Kawaki is being controlled by an external force that has a vendetta against Naruto and uses Kawaki as a tool to carry out the murder.
The battle between Saitama and Cosmic Garou in the fanfiction would have a catastrophic effect on the environment. First, Saitama's initial attacks might send out concussive forces that uproot trees and scatter them like matchsticks. Cosmic Garou's response could involve creating some sort of cosmic vortex that sucks in everything around it, from small pebbles to large boulders. As the fight progresses, the area around them would become a desolate wasteland, with fires raging from the heat generated by their attacks and the air filled with dust and debris, making it difficult for any living thing to survive in the vicinity.
Maybe a fanfic where Naruto and Boruto are pitted against each other in a virtual reality battle. It could be part of a training exercise that gets out of control, and they start using their real skills and emotions, leading to a intense and revealing fight.