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 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. He's always been competitive with Naruto, and he might see Fuu as an obstacle in Naruto's path. Also, some of the more arrogant ninja from other villages, like those who look down on Naruto's boisterous nature and Fuu's different style of fighting, could be rivals.
The intense relationship between Naruto and Sasuke. Their rivalry is full of passion and history. In fanfiction, authors can explore different aspects of it, like what if they had different upbringings but still had the same rivalry.
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.
Often, Sasuke can be a rival for Naruto in many fanfictions during the Chunin Exams. For Ino, Sakura could be a rival as they both have a competitive streak. In some fanfictions, there might be other characters from different villages who also pose as rivals for them during the exams.
One idea could be a story where Naruto and the Lee brothers team up for a special mission. They could face a powerful enemy that requires their combined skills. Naruto's jutsus and the Lee brothers' taijutsu would make for an exciting battle sequence.
Well, in this fanfiction, Naruto might face some ancient warriors who have been awakened for the tournament. These could be warriors with skills that have been long forgotten in the modern shinobi world. There could also be some ninja from clans that are exclusive to the western regions. These clans might have special abilities that Naruto has to figure out how to counter during the battles. And, there could be some foreign fighters who are not shinobi but have their own forms of combat that are equally challenging for Naruto.
A really good 'naruto sasuke rivals fanfiction' to check out is 'Beyond the Rivalry'. In this fic, Naruto and Sasuke are forced to team up against a new, powerful enemy while still maintaining their rivalrous feelings towards each other. It shows how they balance their personal competition with the need to protect their world.