In a fanfiction where Happy dies, perhaps it could be due to a powerful dark magic spell that was meant for someone else but Happy sacrificed himself to save them. This could lead to a series of events where the Fairy Tail members search for a way to revive him. They could go on a dangerous quest, facing numerous challenges and meeting new characters who might have the key to bringing Happy back to life.
Maybe Happy dies in a battle against a new, extremely powerful enemy. His death causes a rift between the Fairy Tail members as some blame others for not being able to protect him. This internal conflict could be the main focus of the fanfiction, while also showing flashbacks of Happy's happiest moments with the guild, like when he first joined and all the pranks he played.
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.
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.
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 It could lead to changes in the guild's dynamic, with some members perhaps becoming more withdrawn while others might be driven to seek revenge if their deaths were due to an enemy's actions.
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.
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 interesting idea could be exploring Happy's final moments in a very emotional way. Maybe it shows his selfless acts even in the face of death.
One idea could be that in their human forms, Happy and Carla are part of a detective agency in Magnolia. Happy is the enthusiastic junior detective while Carla is the more serious and observant one. They solve mysteries related to magic items that have gone missing, often running into other Fairy Tail members who unknowingly hold clues.
One idea could be that Happy, in human form, discovers a new magic power that only humans can possess. He then has to learn how to control it while also dealing with the strange human emotions that come with it.
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.