He might be a character based on a self - insert by the fanfiction author. The author could have projected themselves or someone they know into the Fairy Tail universe, and this character has the qualities that make him seem like 'the one'. This could include having a special understanding of the magic in the world, or a unique relationship with the existing Fairy Tail characters. It could also be that this 'he' is a character who has been wrongly underestimated in the original Fairy Tail, but in the fanfiction, he is revealed to be 'the one' with hidden powers or a great destiny.
It could be about a male character in a Fairy Tail fanfiction who is seen as 'the one' for a particular role, like the one to save the day, or the one to win a certain character's heart.
One challenge Naruto might face is getting used to magic. Since he's used to chakra - based powers, the concept and use of magic could be quite confusing at first. Also, the rules and culture of Fairy Tail might be different from what he's used to in Konoha. He'd need to adapt to the guild's way of doing things.
The condition could be that the main character has to give up their magic for a period of time. This would create a lot of interesting plot twists as they'd have to rely on their wits and the help of their friends in the Fairy Tail guild instead of their magical abilities.
Perhaps an older, disgruntled member of Fairy Tail who has some hidden grudge against Natsu. Maybe Natsu unknowingly got in the way of this member's plans or achievements in the past, and now they see an opportunity to get revenge. They might use their knowledge of Natsu's fighting style and weaknesses to launch a surprise attack within the guild.
Fans of Fairy Tail who like to see alternative and darker storylines might be interested.
I think Luffy would befriend Lucy first. Lucy is kind - hearted and she has a similar sense of adventure as Luffy. She also has a collection of celestial spirits which would be really interesting for Luffy. He might see her as a valuable ally and friend right away. And with Lucy being one of the more approachable members in Fairy Tail, it's likely that they'd start a conversation and become friends quickly.
He could have an interesting relationship with Natsu. Natsu is known for his hot - headedness and love for a good fight. The Hulk, with his anger - based power, could be a great sparring partner for Natsu. They might start off as rivals, constantly trying to outdo each other in terms of strength. Or they could become friends who understand each other's need to let out their pent - up energy. He could also have a relationship with Lucy. Lucy might be a bit intimidated by him at first, but as she gets to know him, she could see his good side and try to help him control his anger. Maybe she could use her celestial spirits to calm him down in some situations.
Well, Naruto could bring his unique sense of justice to Fairy Tail. Since he has always been about protecting his friends and village, he would likely jump into any situation in Fairy Tail where the guild or its members are in danger. His Rasengan could be a very powerful attack against the various foes in the Fairy Tail world. And he could learn some magic from the Fairy Tail wizards too, like how to control magical elements which would be a whole new experience for him.
If Shirou joins Fairy Tail in fanfiction, his character might develop a greater sense of belonging. He has always been on his own journey, but in the guild, he'd be part of a team. This could lead to him being more open about his feelings and past experiences. Also, his combat skills could evolve as he trains with the other members and is exposed to new fighting styles and magical techniques. He might even start to develop his own form of magic - influenced combat, which would be a new and interesting aspect of his character growth.
It could be a jealous Fairy Tail member. Maybe someone who is envious of Naruto's unique powers and the attention he gets from some of the guild members. This person could start spreading lies about Naruto to turn the others against him.