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 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.
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.
Besides Obito, Sasuke could be a main character in this fanfiction. Sasuke, with his own journey of redemption and his understanding of power and the ninja world, could have an interesting dynamic with the reincarnated Obito. Also, Sakura might play a role. She could be the one who tries to heal the reincarnated Obito, both physically if he is injured during his new adventures and emotionally from the traumas of his past life.
Often, Obito is attracted to Kushina's strength and kindness. He might try to impress her with his own abilities. Their interactions could involve Obito being a bit clumsy in his attempts at first. Kushina, on the other hand, might find his eagerness endearing and start to open up to him more. They could share stories about their pasts, their dreams, which deepens their bond.
There are fanfictions where the 'naruto godlike katon' aspect is combined with some sort of hidden family heritage related to the fire element. In these stories, the character not only has a powerful katon ability but also discovers a deeper connection to their family's past and the secrets of the katon jutsu that have been passed down. This often leads to them becoming even more powerful and respected in the Naruto universe.