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.
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.
Definitely Luffy and Natsu. They both have an insatiable appetite and an unwavering determination. They would get along really well, with Luffy being amazed by Natsu's fire magic and Natsu being interested in Luffy's rubber powers. Another interesting pairing could be Zoro and Erza. They are both incredibly strong and skilled swordsmen (swordswoman in Erza's case). They could exchange sword - fighting techniques and have some epic battles together.
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 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 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.
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.