One idea could be a crossover where characters from Fairy Tail and One Piece meet. For example, Natsu from Fairy Tail could team up with Luffy from One Piece to fight a common enemy. They could be on an island that has elements of both the Fairy Tail and One Piece worlds, like a magical Devil Fruit tree. Another idea is a story where the magic in Fairy Tail is somehow combined with the Haki in One Piece. Maybe a character from Fairy Tail learns to use Haki, or a One Piece character discovers magic. A third idea could be a fanfiction set in an alternate universe where the guilds in Fairy Tail are pirates in the One Piece world, with their own ships and crews, and they search for the ultimate magical treasure.
One idea could be a crossover where the characters from One Piece and Fairy Tail team up to fight a common enemy. For example, the Straw Hats and Fairy Tail guild members joining forces against a powerful dark wizard who also has a connection to the One Piece world's sea devils. Another idea is a character swap. Imagine Natsu from Fairy Tail on the Going Merry with Luffy and his crew, experiencing the adventures in the Grand Line. Or perhaps Robin from One Piece in the Fairy Tail guild, using her knowledge to help with magic research.
One idea could be a crossover where the Straw Hat Pirates and Fairy Tail members team up to defeat a common powerful enemy. Maybe a dark wizard has joined forces with a pirate crew that has a devil fruit user with similar powers to a dark guild's magic.
One idea could be that the Straw Hat crew gets transported to the Fairy Tail world and they have to team up with Fairy Tail mages to fight a common enemy like a powerful dark guild that has somehow obtained Devil Fruits. Another could be a character swap, where Natsu goes on a journey in the One Piece world, using his fire magic instead of Devil Fruit powers.
One idea could be that the Straw Hat crew somehow ends up in the Fairy Tail world and they have to team up with the Fairy Tail guild to fight a common enemy, like a dark magic user who is trying to take over both worlds. Another plot could involve a magic item from Fairy Tail that the One Piece characters need to find in order to unlock a new area in the Grand Line. Or perhaps a character from Fairy Tail accidentally gets teleported to the One Piece world and has to learn how to survive on the high seas while also trying to find a way back home.
Another plot could be that a magic spell in Fairy Tail goes wrong and transports some of the Fairy Tail members to the One Piece world. They end up on an island and have to figure out how to get back home. They meet Luffy and his crew, and they start to exchange skills. The Fairy Tail mages teach the pirates some basic magic, while the pirates show the mages their unique fighting styles. They then team up to take on a powerful pirate who has been terrorizing the island and also has some sort of dark magic ability.
In some fanfictions, the magic system of Fairy Tail and the Devil Fruits in One Piece are combined. This can lead to really unique abilities. For instance, a character might have a Devil Fruit power that is enhanced by Fairy Tail magic, creating a whole new level of power. It allows for a lot of creativity in battles and character development.
One idea could be that the Fairy Tail guild somehow gets transported to the One Piece world. Maybe they end up on an island and have to team up with the Straw Hat Pirates to fight a common enemy like a powerful new pirate crew or a sea king. Another plot could involve a magic item from Fairy Tail being lost in the One Piece world and both groups have to search for it together. This could lead to some really fun interactions between the characters, like Natsu trying to compete with Luffy in eating contests while they are on their quest.
One interesting plot could be that the Fairy Tail guild somehow ends up in the One Piece world. Natsu and his friends could team up with the Straw Hat Pirates to fight a common enemy, like a new type of devil fruit user who has the power to steal magic. They would have to combine their unique abilities, with the Straw Hats' devil fruit powers and the Fairy Tail mages' magic, to take down this powerful foe.
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.