Jiraiya could play an important role. He was Naruto's mentor. He might have some knowledge about such strange transformations from his travels and research. Also, Hinata. Given her deep feelings for Naruto, she would do everything she could to help. She might be more in tune with Naruto's emotions even in his fox form and offer emotional support which is also crucial in such a situation.
Sakura could be one. She's a skilled ninja and also Naruto's friend. She would likely use her medical ninjutsu knowledge to see if there are any physical or chakra - related reasons for Naruto's transformation and try to reverse it. Another one could be Kakashi. He has a lot of experience and knowledge. He might research in the old ninja scrolls to find a solution.
In such a fanfic, Naruto might have to face challenges in communication. Since he's stuck as a fox, he can't use normal human language easily. He may have to rely on body language or simple growls and howls to convey his feelings and needs to his friends. This could lead to a lot of misunderstandings at first.
He could meet Konan. Konan is closely associated with the Hidden Rain Village. Her paper - based jutsu would be an interesting interaction for Naruto. Maybe she could teach him something about her unique style of combat or they could end up in a battle.
Well, obviously Naruto and Hinata as Shinachiku's parents. His friends could be the children of other Naruto characters, like Boruto's friends. Maybe the son or daughter of Shikamaru could be his strategic partner in some adventures. Also, the new generation of the Akatsuki could be potential villains in the fanfic.
Obviously, Kurenai herself could be a central character in the harem. Also, Anko could be a potential member as she is associated with Kurenai and has her own unique personality. Hinata might also be involved as she has a crush on Naruto and could be drawn into this situation due to her connection with Naruto and the overall Naruto - Kurenai scenario.
Kakashi could be a potential swordsman character. He's known for his combat skills. In the harem, Hinata could be there. Her gentle nature would contrast with the action-packed world. Also, Ino could be part of it, using her mind - control jutsu in unique ways to protect the group and be involved in the harem's relationships.
Naruto himself could also be a main character. He could visit the Hidden Star Village and interact with the Hoshikage, getting involved in the village's affairs and helping them solve their problems. This would be an interesting way to incorporate the Naruto we all know into the fanfic.
Well, obviously Naruto and Konan are the main ones. But other characters like Sakura could be part of it. Sakura, being a strong - willed and skilled kunoichi, might be attracted to Naruto's growth and his new - found status in the ninja world. Hinata could also be included. Her long - standing love for Naruto could easily fit into the harem concept, and she could interact with Konan in an interesting way.
Iruka sensei could also be very helpful. He has always been like a father figure to Naruto. He would support Naruto through this strange transformation, offering advice on how to handle the reactions of others in the village and how to adapt to this new situation without losing sight of his ninja goals.
There's also Killer Bee. Since he has a connection to the outside world and is a powerful ninja. Naruto and he could cross paths in Kiri. They might even have some rap battles or just share their ninja experiences. Another character could be Zabuza. Even though he's dead in the original story, in a fanfic, Naruto could somehow encounter his spirit or a young Zabuza - like character and learn some of his swordsmanship skills.
It's possible that a group of students could be the main characters. They all learn from the genius sensei in different ways. One student might be great at physical combat, another at ninjutsu, and they have to work together under the sensei's guidance. Their combined growth and how they interact with each other and the sensei can form the core of the 'naruto genius sensei fanfic'.