One great example could be a story where Natsu from Fairy Tail and Luffy from One Piece team up to take on a powerful villain that threatens both their worlds. In this fanfiction, they might have to learn to combine their unique fighting styles, like Natsu's fire magic with Luffy's Gum - Gum abilities.
Often, there are fanfictions where the worlds of Fairy Tail and One Piece start to merge. The islands in the One Piece world might start to have magical properties and the Fairy Tail guild could have to deal with the chaos of the pirates suddenly showing up in their world. This can lead to some really interesting character interactions, like Erza trying to discipline Zoro or Lucy befriending Nami and sharing their knowledge about different types of treasures.
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.
Blackbeard would probably be very interested in the magic in the Fairy Tail world. He might try to steal some of the powerful magic items or learn magic for himself. He could see the mages as a threat to his plans if they oppose him, but also as potential allies if he can manipulate them. He would likely use his cunning and power - hungry nature to his advantage in this new world.
One possible plot could be that the Straw Hat Pirates end up in the Fairy Tail world due to some magical anomaly. They team up with the Fairy Tail guild to fight a common enemy like a powerful dark wizard who has the power to control the seas as well. Luffy and Natsu could have a friendly competition about who is stronger while their respective crews and guild members learn from each other's fighting styles.
Luffy's stretchy powers could be used in creative ways when combined with Fairy Tail magic. For instance, if he were to stretch around a Fairy Tail mage who was casting a large - scale spell, he could potentially redirect the spell's energy. Natsu's fire magic might be able to make Luffy's rubber burn in a different way than normal, creating new attack possibilities for both of them.
Well, for instance, Natsu's fire magic could potentially interact with Ace's fire - based Devil Fruit powers in interesting ways. They could either combine their flames to create a more powerful attack or compete to see whose fire is stronger. Also, characters like Luffy with his rubber body might be able to dodge the attacks of Fairy Tail mages more easily due to his flexibility. And mages like Gray who can manipulate ice could create icebergs in the middle of the ocean in the One Piece world, which would be a new and unexpected obstacle for the pirates.
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.
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.