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.
Kakuzu might be a rival. Although he is more known for his earth and water techniques, in a fanfiction scenario, he could have developed a strong interest in fire and trained hard to become a master. His ability to use multiple hearts could give him an edge in combat against Naruto. And then there's Madara Uchiha. Even though he has a wide range of powerful abilities, he could also be interested in challenging Naruto for the title of master of fire, just to show his dominance in all aspects of ninjutsu.
One potential rival could be a new character introduced in the fanfiction. This character could be a prodigy in fire jutsu, having been trained in a secretive and ancient fire - using clan. They would be determined to take down Naruto and become known as the true master of fire. Also, Orochimaru, if he were still around and had an interest in fire - based powers, could be a formidable rival. He could try to use Naruto's fire abilities for his own evil purposes or just to prove his own superiority in the ninja world.
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.
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 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 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.
Perhaps an ancient demon that was once sealed away but has now been released due to some unforeseen circumstances. The demon has an affinity for fire and sees the Fire Master as a threat to its plans of world domination. It would constantly try to eliminate the Fire Master and anyone who stands in its way, including Naruto and his friends.
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 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.
Sakura could be one of his apprentices. She has always been smart and determined, so she would be a great student. She could learn from Naruto how to use her chakra in the smithing process to create even better tools.
Rock Lee could be an interesting rival. His extreme dedication to training and his unique fighting style would make for some great battles against a female Naruto. Tenten, too, could be a rival as she is skilled with weapons and would want to show that she is better than a female Naruto in combat. Another possible rival could be Hinata. She might be more confident in this fanfiction and see the female Naruto as someone she needs to overcome in order to prove herself, especially in the area of ninjutsu.