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.
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.
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 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.
Another focus could be on their powers. In human form, their magical abilities might manifest in new ways. Maybe they can combine their powers in a unique way that was not possible before. They could use these new powers to protect the human world from threats that are related to the magic world of Fairy Tail.
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.
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.