It could be a rogue ninja who was once a student of a great fire - style master but turned evil. He wants to use the power of the 'fire fist' to destroy all the ninja villages and rule over the world.
A great fanfiction concept would be a crossover where Naruto is taught the art of the fire fist by a character from another anime. This could lead to new and exciting battles as he combines his existing skills with the new fire fist power.
Well, perhaps in this 'fire fist naruto fanfiction', the 'fire fist' is actually an ancient power sealed away long ago. Naruto discovers it while on a mission in an old, forgotten temple. He then has to deal with the fact that there are dark forces also after this power, and he has to master it quickly to protect his village. It could lead to some really cool battles and new friendships as he teams up with other ninjas to keep the power safe.
Sometimes, an original character created specifically for the fanfiction could be a main character. This character could be a mentor - like figure who teaches Naruto about the dragon fist. They might have a deep knowledge of the technique's origin and its true potential, guiding Naruto on his journey to master it.
I don't know specifically as it depends on the fanfiction. It could be Naruto and his friends if the 'ninth fist' is something they encounter or develop.
Rock Lee could be a great main character. He's known for his taijutsu skills, and adding the Explosive Fist to his repertoire would make for an exciting story. He could use it to overcome his limitations in ninjutsu and genjutsu and prove himself even more as a powerful ninja.
If Naruto had fire fist powers in fanfiction, his combat style would be completely different. He could use the fire fists for both close - range attacks and long - range by shooting out fire projectiles. This would make him a more versatile fighter.
I'm not entirely sure as there could be many interpretations. It might be a fan - made story that involves a unique fighting style or technique related to Naruto, perhaps a new form of the 'ninth fist' that the author has created within the Naruto universe.
Naruto is surely the central character in this fanfiction. Besides, characters like Kakashi could be involved. He might be Naruto's mentor in controlling and understanding his new 'king of fire' powers. And villains from the Naruto universe might also appear. They could be envious of Naruto's power and try to take it away or use it for their own evil purposes. So, a variety of characters from Naruto's world could be main characters in this fanfiction depending on the plot.
It could be the descendants of the original Naruto characters. For example, Naruto's son might be one of the main characters, carrying on his father's legacy and the will of fire.
It's possible that the main characters could also include some of the antagonists from the original series. Maybe they are behind the cause of the fire and ash, or they take advantage of the chaos to further their own goals. Naruto and his allies would then have to face these enemies while also dealing with the desolate environment.