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.
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.
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.
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.
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 some fanfictions, Naruto's pain might manifest through his actions. For example, he could become withdrawn and isolate himself from his friends. This is because Naruto has always been a social person, so any deviation from his normal behavior could be a sign of his internal pain.
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.
I would recommend 'Pain's New Beginning'. In this fanfiction, Pain gets a chance to relive his life after a strange encounter. It has some really cool plot twists that keep you hooked. It also explores his relationships with other characters in new ways.