Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
Well, in a lot of fanfictions about Luffy vs Katakuri, Luffy's growth is depicted quite vividly. His growth can be seen in his improved observation Haki. He starts to anticipate Katakuri's attacks more accurately, which is a huge step forward. He also becomes more creative with his attacks. Instead of just using his standard punches and kicks, he combines different moves and even creates new ones on the fly. This shows that he's evolving as a fighter and not just relying on his basic skills. Another aspect is his mental growth. He faces a very tough opponent in Katakuri, but he doesn't give up. His resolve gets even stronger, which is an important part of his overall growth in these fanfictions.
One possible plot could be that there is a rift in the multiverse that brings Goku and Luffy together in a strange new world. They initially misunderstand each other and start to fight. As the fight progresses, they both realize that there is a greater threat at hand that is causing the rift. So, they team up, with Goku teaching Luffy some of his combat techniques and Luffy showing Goku the value of friendship and loyalty in a different way.
The "Negative Fruit" in One Piece was one of the superpowers that Luffy encountered for the first time in the story. The scene where Luffy and his gang fought against the "Negative Fruit" appeared in episode 598,"Luffy and Luffy!" Adventures in Two Worlds.