Even some of Naruto's former allies could turn into rivals in fanfiction. Maybe due to a misunderstanding or a new power - hungry ambition, they start to compete with Naruto for the title of master of the elements. This would add an interesting twist to the story as Naruto has to deal with not only external threats but also internal strife among his once - close comrades.
Sasuke could be a rival in this fanfiction. Sasuke has always been strong in fire - based jutsu himself. He might see Naruto's claim as the master of fire as a challenge and want to prove his own superiority. Another could be some of the members from other ninja villages who have also mastered powerful fire techniques. For example, a ninja from the Hidden Rock Village who has a unique and powerful fire - based kekkei genkai.
There could be a group of rivals rather than just one. A team of young Johto trainers who idolize the previous Johto League champions. They see Ash as a threat to their dreams of following in their idols' footsteps. They might gang up on Ash during battles or try to sabotage his journey in more subtle ways, like spreading false rumors about him in the Pokémon Centers. This would force Ash to not only focus on his Pokémon training but also on his reputation management.
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.
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 Naruto's main rival in the 'naruto is a gamer' fanfiction. Just like in the original Naruto story, Sasuke is always competing with Naruto. In the gaming world, they might compete for the highest score or to be the first to complete a difficult level. Their rivalry could drive them both to improve their gaming skills rapidly.
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.
Sasuke could be a major rival. Even if Naruto is a child genius, Sasuke's natural talent and his drive for revenge could put him in direct competition with Naruto. They might constantly try to outdo each other in training and in battles.
Jiraiya could play a crucial role. As Naruto's mentor, he might have left behind some hidden teachings or prophecies that Naruto discovers in the fanfiction. These could guide Naruto on a path to unlock new powers or understand the secrets of the ninja world better.
Sasuke could be a rival. Even though they are friends, Sasuke is also a very powerful ninja. He might see Naruto's progress as a Konoha Jutsu Master and feel the need to compete to prove himself. Another could be Neji. Neji is highly skilled and might not be entirely happy with Naruto getting so much attention as a jutsu master.
In fanfiction, Ash might discover a special water - type stone that enhances his bond with his water - type Pokemon. This stone could have the power to boost their abilities and his own understanding of them. With this new advantage, he enters water - type Pokemon tournaments and gradually builds his reputation as a water master. He also shares his experiences with his friends, who support him in his journey.