One interesting aspect could be the unique character interactions. In fanfiction, the relationships between the characters from 'Fairy Tail' can be explored in new and unexpected ways. For example, Happy's character might be developed further in a roasting scenario, showing a different side of his personality. Maybe he gets into comical arguments with other characters, which can add a lot of humor to the story.
Obviously, Happy is one of the main characters since he is in the title. He's a cute and often comical character in 'Fairy Tail'. Natsu would also be likely to be involved. He's Happy's best friend and they are always together in the original series. Their interactions would be a natural fit for a roasting scenario. Lucy might also be part of it. She has a strong personality and could easily get into a roasting battle with the others.
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.
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.
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.
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.
For Natsu, it would be devastating. Happy has been his constant companion since they were young. Natsu would likely be filled with rage and grief, which could potentially lead him to seek revenge against whoever or whatever caused Happy's death.
Maybe they leave because they want to search for answers about their pasts. Happy might want to know more about his origins, and Lucy could be seeking something related to her family's history.
Happy's simple - mindedness often leads to humorous situations. He might misunderstand a complex plan in the guild and go off on his own tangent, creating chaos. For example, if the guild is planning a surprise party for someone and Happy misinterprets the instructions, he could end up ruining the surprise in a really comical way.
In some fanfiction, a human who is an artist finds a fairy tail. The fairy in the tail has the power to bring art to life. The human and the fairy work together to create a beautiful, magical art gallery. They invite other humans and fairies to see their creations, spreading happiness and harmony between the two worlds.