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.
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.
One great 'Katon Naruto fanfic' could be 'The Blazing Trials of Naruto'. In this fic, Naruto discovers a new form of Katon jutsu through intense training. It's really exciting as it shows his growth not just in power but also in character.
Another option is Reddit. There are Naruto - related subreddits like r/NarutoFanfiction. You can post a request asking for 'Katon Naruto fanfic' and the community might be able to point you to some great reads or even share their own creations.
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.
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.
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.
Naruto could be one of his allies. Naruto has this knack for seeing the good in people and would probably be willing to give Obito a chance to redeem himself. Also, Sakura, with her skills as a medic - ninja, could be an ally. She might help heal Obito if he was injured during the war and see the potential in him to be a force for good.