In a fanfiction, we could have a scenario where an enemy uses some ancient jutsu to temporarily take control of the One Tail within Naruto. Naruto then has to fight an internal battle to break free from this control. Meanwhile, his friends try to find a way to help him from the outside. This could lead to a lot of character development for Naruto as he battles his own power.
One idea could be exploring the relationship between Naruto and the One Tail's spirit. Maybe they can communicate in a dream - like state, and the One Tail gives Naruto some unique insights about his power and his role in the ninja world.
A plot could involve a powerful dark force threatening both the Naruto and Fairy Tail universes. This force could be something that neither ninjutsu nor magic can easily defeat. So, the characters from both worlds must come together. The GMG could be a platform where the first interactions occur. Maybe some Naruto characters enter the GMG under false pretenses to investigate this new threat that seems to be related to the event.
One idea could be that Naruto characters end up in the Fairy Tail world and have to participate in the Grand Magic Games. They could show off their ninja skills which would be a novelty there. Another could be a team - up where Fairy Tail mages and Naruto ninja join forces to fight a common enemy that threatens both their worlds, like an ancient evil that can travel between dimensions.
One idea could be a crossover where a powerful Naruto villain somehow ends up in the Fairy Tail world and causes chaos. The Fairy Tail mages would have to use their unique magic to combat the ninja - like abilities of the Naruto character.
One idea could be that the Fairy Tail guild travels to the Naruto world somehow. They could team up with the ninja to fight a common enemy like a powerful dark magic user who has also invaded the ninja world. Maybe Natsu's fire magic could be compared and combined with Sasuke's fire jutsu in some unique ways during battles.
Maybe there could be a plot where a powerful villain from the Naruto universe, like Orochimaru, teams up with a dark mage from Fairy Tail. The combined threat forces the heroes from both worlds to work together. The Fairy Tail members would be amazed by the unique abilities of the ninja, like Sasuke's Sharingan and its various powers. In turn, the Naruto characters would be intrigued by the wide array of magic in Fairy Tail, such as Erza's Re - Quip 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.
One idea could be a crossover where Naruto characters learn magic from Fairy Tail. For example, Naruto might learn fire magic and combine it with his Rasengan. Another idea is that the guilds in Fairy Tail and the villages in Naruto have to team up to fight a common enemy, like an ancient dark force. And perhaps some of the Fairy Tail wizards could teach the ninja in Naruto about the power of friendship and how it can enhance their abilities.
One idea could be a character having the 'All For One' quirk in the Naruto universe and using it to steal other ninja's unique abilities. For example, they might target a Sharingan user to gain that power. Another plot could involve a group trying to stop this 'All For One' - like - character from disrupting the balance of power in the ninja world. And perhaps the character with 'All For One' has a hidden motive related to an ancient prophecy in the Naruto world.
Well, a really cool plot might be that Orochimaru, always interested in new powers, somehow gets his hands on the 'All for One' concept. He could start using it to assimilate the powers of different beasts, like the tailed beasts, in a new and different way than just sealing them. Or perhaps a young genin discovers they have the 'All for One' ability. They initially use it to help their friends by taking on some of their weaknesses, but then it attracts the wrong kind of attention from the Akatsuki, who see it as a powerful tool for their own goals. Also, a time - travel plot where a character from the future with the 'All for One' ability comes back to the Naruto world to change the course of history, either for good or bad.