Sasuke could be a potential rival. He is highly talented and always striving to be stronger. His determination and different fighting style would make for an interesting contrast to the Nara prodigy.
Another possibility could be Sakura. Although she was in love with Sasuke, in some fanfictions, the storylines can be altered. Sakura is a strong and intelligent character, and she could potentially have a daughter with Naruto in a different narrative where their relationship develops in a new way.
Sasuke could be a major rival. Since Sasuke also has strong fire - based abilities, he might see Naruto's growth as a Katon master as a challenge to his own skills. Another potential rival could be a new character introduced in the fanfiction, a prodigy from another village who is also focused on mastering Katon. There could also be older, more experienced ninja within the Leaf Village who might initially doubt Naruto's ability to be a true Katon master and thus become rivals in a sense as they test his skills.
Well, obviously the blind prodigy themselves is a key character. There could also be original characters created specifically for the fanfiction. For example, a rival who also has some sort of disability but uses it in a different way. Another potential character could be a healer who helps the blind prodigy learn to use their other senses better. And of course, existing Naruto characters like Naruto, Sasuke, Sakura, and Kakashi are likely to make appearances. They could interact with the blind prodigy in various ways, like Naruto being a source of motivation or Sasuke being a foil to the blind prodigy's more positive attitude.
Sasuke could be a potential rival. In many Naruto fanfictions, Sasuke is highly skilled in ninjutsu and sharingan - based genjutsu. His rivalry with Naruto, if Naruto becomes a genjutsu master, would be intense. Another could be Itachi. His mastery of genjutsu, like Tsukuyomi, would make him a formidable opponent for Naruto in the genjutsu realm.
Another potential rival could be a young prodigy named Lily. She has a diverse team of Pokemon that she has trained from a very young age. Lily is very confident in her abilities and constantly tries to outdo Phoebe in battles. She might also have some unique training methods that Phoebe has to figure out in order to defeat her.
The Akatsuki could be potential enemies. They might see the Mokuton Prodigy as a threat or a valuable asset to capture. Their diverse powers and motives would make for interesting confrontations with the prodigy.
Sasuke could be a rival. He's always been highly skilled and competitive. His Sharingan could potentially see through some of the henge master's henges, creating a constant challenge for the henge master to come up with more advanced henges.
Sasuke could be one. He already has a strong sense of focus and discipline, which could be transferred to his musical pursuits. He might be a prodigy in playing some traditional shinobi musical instruments and use the music as a way to enhance his Sharingan abilities.
It could be someone like Rock Lee. He was a hard - working prodigy in taijutsu, but often got overshadowed by the more flashy abilities of others. Maybe in this fanfiction, he's completely forgotten until he does something extraordinary to get noticed again.
Perhaps it could be a new character created specifically for the fanfiction. A young ninja from a minor clan who showed extraordinary talent in a unique form of jutsu. But because their clan was small and not very influential, and the larger clans felt threatened by this new talent, they managed to get the village to abandon this prodigy.