It could be Naruto himself who discovers and uses the Katon Hijingakure no Jutsu in the fanfiction. Since he's always exploring new jutsus and pushing his limits.
Well, in this fanfiction, the Katon Hijingakure no Jutsu might have unique characteristics such as a more intense and hotter flame compared to the regular Katon techniques. Maybe it also has some special shape or movement pattern that is distinct from other fire - based jutsus in the Naruto universe.
One possible plotline could be that Naruto discovers this new jutsu during a difficult battle. He then uses it to turn the tide of the fight, much to the surprise of his opponents and allies alike.
I'm not sure of specific 'Naruto said Hailingakure no Jutsu' fanfictions off the top of my head. However, you could also check out Archive of Our Own (AO3). It has a large collection of fan works. Try using different search terms related to Naruto and this jutsu to narrow down the results.
There could be a jutsu that allows for instant teleportation across long distances. This jutsu might be a new discovery in the fanfiction world, perhaps created by a character who has been experimenting with space - time ninjutsu. They might use it to quickly get in and out of battles or reach far - off places in a blink.
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.
I'm not sure which one is the most popular specifically, but there might be some on platforms like FanFiction.net or Archive of Our Own. You can search for 'naruto godlike katon' and look at the ones with high ratings and a lot of reviews.
He might learn a time - traveling jutsu. This would be really cool as it could allow him to go back and change certain events in the Naruto world, or gain knowledge from the past to use in the present battles.
In Naruto fanfiction, cheat codes can drastically change the use of jutsu. They might allow a character to instantly master a jutsu that usually takes years of training. For example, a cheat code could give Naruto the ability to use the Rasengan without any training from Jiraiya. It's a fun concept that adds an interesting twist to the Naruto universe.
A 'Solar - Flare Rasengan'. Naruto could combine his wind - style chakra with some kind of solar - based energy he discovers. This jutsu would be like a super - charged Rasengan that emits a blinding light as well as powerful wind - based damage.