Yes, in certain fanfictions Naruto is immune to fire. This can add an interesting twist to the story. For example, if he's facing a powerful fire - using enemy, his immunity gives him an advantage. It also allows for new plot developments and character interactions that wouldn't be possible in the original story.
One way could be through a new type of chakra transformation. For example, if Naruto discovers a way to convert his normal chakra into a special 'fire - repelling' chakra. Another way might be some sort of ancient curse or blessing that makes him immune.
One implication is that he becomes a more formidable opponent in battles. Enemies who rely on poison - tipped weapons or poison - based jutsus would be at a disadvantage against him.
In certain fanfictions, Naruto's immunity to poison might be a result of his training with a hermit or some ancient sage. They could have taught him special techniques or given him elixirs that made his body resistant to poison over time. For example, he might have had to endure harsh trials in a poisonous environment, and his body gradually adapted to it, leading to complete immunity.
It could be related to some kind of magic protection. In the wizarding world, there are various protective spells and enchantments. Maybe Harry has acquired or was born with a special protection that makes him immune to allure in this fanfiction.
It also means he is more reliable for his friends. Since he is immune to allure, his friends can trust that he won't be swayed into making bad decisions that could endanger them or their cause. He can be a stable and trustworthy figure in the fanfiction's world. For example, when facing a difficult situation where there are temptations to betray the group, he will firmly stand with his friends.
In RWBY fanfiction, the immune response could be depicted in various ways. It might be related to how the characters deal with a new type of Grimm toxin. For example, some fanfics could show a character having an enhanced immune response due to their unique Aura training, which helps them fight off the negative effects more quickly than others.
Often, it's portrayed through the characters' ability to resist certain negative effects. For instance, if there's a disease - like condition caused by some dark magic in the fanfic, characters with stronger Auras might show a better immune response by not getting as sick or recovering faster.
Well, in fanfiction, just about anything can happen. Since it's not bound by the strict canon of either the Naruto universe or the Avatar universe (where fire bending comes from). Writers can give Naruto the power to fire bend. They might do this to create new plotlines, like Naruto using fire bending to fight a new kind of enemy or to master a new form of combat. It's all about creativity in the world of fanfiction.
There's also the 'Inferno Storm Jutsu' in some fanfictions. It creates a large - scale storm made entirely of fire. The fire in this storm has different intensities, with the center being the hottest and most destructive. It can cover a large area, making it a great offensive move against groups of enemies.