Another interesting plot could be that Happy, as a human, decides to go on a journey to find his true identity. He might have some memories of his Exceed life, but he wants to understand more about his human side, like where he came from and if he has any family in the human world.
Perhaps Happy in human form could get involved in a love triangle. He falls in love with a human girl, but there's also another character who has feelings for him. This would create a lot of drama and interesting interactions.
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 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.
A really interesting plot could be Acnologia in human form forming an unlikely alliance with the Fairy Tail guild. He could be on a quest to redeem himself for his past dragon - related atrocities and the guild helps him in some way. This could lead to many adventures where he has to use his human skills and not just his dragon powers.
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.
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.
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.
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 idea could be that in fanfiction, Natsu's Dragon Form could have unique fire abilities. For example, he might be able to create different shapes of fire, like a dragon made entirely of fire that can attack enemies on its own. Another could be that his Dragon Form has a connection to an ancient magic that gives him the power to heal other dragons or dragon slayers.
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 do a time - travel fanfiction. Have a character go back in time to when the Fairy Tail guild was first founded and see how they interact with the founders. Or, write a story where the characters swap powers. So, Erza would have Lucy's celestial magic and Lucy would have to use Erza's requip magic. This would lead to a lot of comical and also challenging situations for them. Also, you could write a fanfic about the guild taking on a mission in a completely new and strange land, filled with magic they've never seen before.
One idea could be exploring an alternate universe where the main characters have different magic abilities. For example, Natsu could have ice - based magic instead of fire. Another idea is to focus on the backstories of the minor characters in more depth, like how they came to join Fairy Tail. A third idea would be to create a time - travel fanfiction where characters go back in time to change a significant event in the Fairy Tail world.