In a fanfiction where Happy dies, the overall mood of the story would turn darker. Fairy Tail has always had elements of hope and friendship, but Happy's death would be a major blow. It could lead to the discovery of new powers within the remaining characters as they try to fill the void left by Happy. For instance, some of the younger members could step up and show newfound courage in the face of this tragedy, changing the power structure within the guild.
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.
The Fairy Tail guild's dynamic would be completely different. Happy was like a little ball of sunshine in the guild. Without him, the guild hall would seem much quieter and gloomier. The other members who were close to Happy, like Wendy and Carla, would also be deeply affected. Wendy might struggle to heal the emotional wounds of the guild members, as she's used to healing physical ones. And Carla could become more protective of the remaining friends, fearing more losses.
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.
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.
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 It could lead to changes in the guild's dynamic, with some members perhaps becoming more withdrawn while others might be driven to seek revenge if their deaths were due to an enemy's actions.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.
His loyalty would still be a big part of his personality, but it could be expressed differently. Instead of just following Natsu around like he did as an Exceed, he might try to support Natsu in more strategic ways as a human, using his newfound human intelligence.
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.
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.
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.
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.