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.
In these fanfictions, Naruto's character is often based on his canonical self, which is full of energy and a bit of a goofball at times. Menma, on the other hand, may be a bit more mysterious or have a different backstory that shapes his character. But they can both have a strong sense of justice. Naruto has always fought for what he believes is right in the main series, and Menma can be written to have a similar moral compass in fanfiction. They may also both experience growth throughout the story, learning from their mistakes and becoming better ninja.
Sakura could be really interesting. She might be confused at first by Menma's presence, thinking he's some kind of impostor Naruto. But as she gets to know him, she could see his unique qualities and might even develop a friendly rivalry with him. Another interesting character could be Kakashi. He would be observing Naruto and Menma's interactions closely, maybe even giving them some special training to help them work together better.