It could be about Naruto having some sort of connection to Pain. Maybe in this fanfiction, Naruto is somehow related to Pain in a way that wasn't in the original story, like being his son in an alternate universe. This could lead to a whole new set of storylines, with Naruto perhaps having to deal with the legacy of Pain while also following his own path as a ninja.
In this fanfiction, it could be that Naruto being the son of Pain gives him access to unique powers or knowledge. Since Pain had powerful Rinnegan abilities, Naruto might inherit some form of that power. This would change the power dynamics in the Naruto universe. He could use these new - found abilities to either continue Pain's goals (if he was swayed by Pain's ideas) or to oppose them and try to redeem his 'father's' name by using the power for good.
Obviously, Naruto would be one of the main characters as it's about him being the son of Pain. Pain himself would also be a crucial character, even if he might not be physically present all the time. And characters like Sasuke, Sakura, and Kakashi would still likely play important roles as they are Naruto's friends and mentors in the original story and would have to interact with Naruto in this new, changed scenario.
It's hard to say precisely without more context. 'naruto son of fanfiction' could be about a lot of things. It could be a story where the focus is on the relationship between Naruto and his son in a fan - created plot. This might include the son's journey to discover his own identity, his training, and how he copes with the legacy of his father. It could also involve new villains or allies that are unique to this fan - made story, and perhaps a whole new arc that revolves around the son's role in the ninja village.
One possible cause could be his sense of failure. Naruto always wants to protect his friends and village, so if he fails in a mission or is unable to save someone, it can cause him great pain.
In many fanfictions, Naruto might use new jutsus or a deeper understanding of his own power. Maybe he taps into a more powerful form of the Nine - Tails' chakra and combines it with his Sage Mode in a unique way to overpower Pain.
His fighting style could become more aggressive. Without the fear of pain, he might be willing to use more extreme techniques that would otherwise be too dangerous because of the pain they cause to the user. He could push his body to the limit more often.
I'm not sure specifically as it's a fan - made concept. But it might be a crossover where Naruto is somehow related to Ozai from Avatar: The Last Airbender in a fictional scenario. Maybe it involves Naruto having some of Ozai's characteristics or being in a situation related to the Avatar world.
I'm not entirely sure as I haven't read this specific fanfiction. But it might be a story that combines elements of the 'Naruto' universe with the idea of Naruto being the'son of Gondor' which could involve some sort of crossover between the Naruto world and the lore of Gondor from Middle - earth. Maybe it has Naruto possessing some of the traits or powers associated with Gondor.
I'm not sure specifically as it depends on the author's creativity. It might be a crossover where Naruto is somehow related to Alduin. Maybe it's a story that combines the Naruto universe with the lore of Alduin, perhaps exploring Naruto having special powers or a destiny related to Alduin.
I'm not sure specifically as there could be many interpretations in such a fanfiction. It might be a story where Naruto has some connection to a 'hollow' concept, perhaps borrowing elements from other fictional universes. It could involve Naruto having unique powers or a different origin story related to the 'hollow' idea.
I'm not sure specifically what that particular fanfiction is about as there could be many different interpretations. It might involve Naruto having some sort of connection to Gaea, perhaps in an alternate universe where Gaea is his mother and it could explore how his character develops with this new origin story, like new powers or relationships.