Another possibility is an accident within the story. Perhaps during a chaotic battle or a magical experiment gone wrong, Happy could be caught in the crossfire or affected by some unforeseen magical consequence. This would be a tragic turn of events for the usually cheerful and helpful Exceed.
Perhaps it's a matter of a powerful enemy from Fairy Tail's lore recognizing Naruto as a significant threat and using a very underhanded or extremely powerful attack on him. Since Naruto might not be fully aware of all the rules and powers in the Fairy Tail world, he could be caught off - guard. And in fanfiction, the writer might want to create a very dramatic and unexpected death for the sake of a unique story, so they come up with a scenario where Naruto dies in the Fairy Tail world.
One idea could be that Happy's death shatters the Fairy Tail guild. Natsu, being his closest friend, goes into a deep depression. The guild loses its usual cheer and the members struggle to come to terms with the loss. They might hold a grand memorial for Happy, and throughout the fanfiction, it could show how each character copes in their own way, like Lucy writing in her diary about all the memories she had with Happy.
If Happy dies, Natsu's power might be affected. Happy has always been by Natsu's side, and his death could cause Natsu to lose some of his confidence. For example, Natsu might not be as reckless in battles without Happy's support. The story could then shift to Natsu trying to regain his former self while also seeking revenge on whoever caused Happy's death.
If Happy and Lisanna die in a Fairy Tail fanfiction, it would have a huge emotional impact on the other characters. Natsu, who is very close to Happy, would be devastated. He would likely go into a state of rage or deep depression. The guild as a whole would also be in mourning. It could lead to changes in the guild's dynamic, with some members perhaps becoming more withdrawn while others might be driven to seek revenge if their deaths were due to an enemy's actions.
In fanfiction, Naruto could become the founder of Fairy Tail by having his unique ninja skills and values influence the creation of the guild. Maybe he travels to the Fairy Tail world and decides to gather like - minded mages. His leadership skills, learned from being a ninja, would help him in setting up the guild's rules and training methods.
Well, in fanfiction, anything can be twisted for the sake of creating drama. Maybe the author wanted to explore a really dark and unexpected scenario. The writer might have thought that by having Naruto betrayed in a Fairy Tail - related setting, it would attract readers who are into crossovers and angst. It could be due to some kind of power struggle or misinformation within the fictional world created in the fanfic.
It could be that the fanfic writer is trying to combine the two universes in a rather dark way. In Fairy Tail, there are all sorts of magic and temptations. If a character from Naruto's world gets involved with someone from Fairy Tail, they might be easily cheated on. For example, if a Fairy Tail character has a magic ability to manipulate emotions, they could use it to deceive Naruto's character.
In some fanfictions where Erza dies, it could lead to a dark and tragic turn for the Fairy Tail guild. Her death might cause the other members to question their strength and purpose. For example, Natsu might be consumed by a desire for vengeance against the ones who caused her death.
In the Fairy Tail world, Naruto would bring his unique ninja abilities. For example, his shadow clone jutsu could be used in creative ways during battles or to perform tasks. He would likely befriend Natsu because they both have a passion for fighting and protecting their friends. Naruto could also introduce new combat styles to the Fairy Tail guild, which might change the way they approach battles.
Well, the plot could involve Natsu being the sole survivor of a great catastrophe that wipes out the rest of Fairy Tail. He might then embark on a journey to revive the guild. He could search for lost magic or artifacts that could bring his friends back. Maybe he'd face off against new and powerful enemies who were responsible for the destruction in the first place.