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.
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.
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.
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 It could lead to changes in the guild's dynamic, with some members perhaps becoming more withdrawn while others might be driven to seek revenge if their deaths were due to an enemy's actions.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 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.
One of the funniest scenarios could be when Happy tries to 'help' Natsu cook but just ends up making a huge mess. He might accidentally add too much of an ingredient or knock over pots while flying around the kitchen in excitement.
In some fanfictions where Lucy dies, it often leads to a dark and somber turn for the other characters. Natsu, for example, might go into a deep rage or depression. Her death could be a catalyst for the guild to seek revenge against the force that caused it. It might also result in the guild falling apart for a while as they struggle to come to terms with her loss.
You could write about an alternate reality where the Fairy Tail guild is cursed and starts to die out. In this fanfiction, Gray might struggle with his ice - make magic as the curse affects his body temperature regulation. And Erza could lead the remaining members in a desperate search for a cure. They might encounter old enemies who offer help but with ulterior motives.
It could lead to new and interesting storylines. Fanfiction often explores alternative scenarios and character developments not seen in the original.
Well, in a Fairy Tail fanfiction with Saitama, he would probably be quite a comical addition at first. The Fairy Tail members would be astounded by his power. He might not fully understand the magic - based society at first, but his sheer strength would earn him respect. And he could also form some interesting friendships. For example, he could bond with Natsu over their love for a good fight, although their fighting styles are completely different. Saitama's no - nonsense attitude might also influence the more hot - headed members of Fairy Tail to be more focused.