Well, perhaps a spell gone wrong in Fairy Tail transports Naruto to their guild. The guild members are initially suspicious but then realize his power. The harem aspect could start when Naruto helps the female members with their personal problems. For instance, he helps Levy find a rare book she's been searching for. His selfless acts make the likes of Mirajane, who is used to seeing all kinds of people in the guild, develop a soft spot for him. And as he continues to interact with different female characters, they all gradually fall in love with him, leading to a harem situation.
One idea could be that Naruto somehow ends up in the Fairy Tail world and is mistaken for a powerful wizard. The female characters in Fairy Tail start to develop feelings for him due to his unique ninja skills and kind heart. For example, Lucy might be intrigued by his ability to summon shadow clones and see it as a form of celestial spirit magic. Another plot could involve a magical artifact that links the two worlds, and Naruto and the Fairy Tail characters have to work together to protect it from a common enemy. This would bring them closer and lead to the development of a harem situation as the female characters from both universes start to fall for Naruto.
A cool plot might be that there's a tournament where ninjas from Naruto's world and wizards from Fairy Tail compete. Naruto participates and shows off his amazing jutsus. The Fairy Tail girls are impressed and start to vie for his attention. Erza could see his determination in battle and be attracted to him, while Wendy might admire his strength in protecting others. As the tournament progresses, a dark force emerges that threatens both worlds, and together they have to fight it. This shared experience cements their relationships and forms a harem around Naruto.
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.
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 a crossover where the magic from Fairy Tail combines with the ninja skills in Naruto. For example, Naruto characters could learn magic and Fairy Tail mages could learn ninja techniques. In the harem aspect, the main character, say Naruto, could have female characters from both series attracted to him because of his newfound powers and his unique personality traits.
One plot idea could be Naruto, with his sage mode powers, getting transported to the Fairy Tail world. He could team up with Natsu and the gang to fight a common enemy. Maybe they have to battle a dark guild that's using some kind of jutsu - like magic combination. Another idea is that Naruto's tailed beasts' chakra could be mistaken for some extremely powerful lost magic in the Fairy Tail world, causing all sorts of chaos and adventure as different groups try to either harness it or destroy it.
One idea could be to have Naruto characters learn magic from Fairy Tail. For example, Naruto could learn Fire Magic, which would be an interesting twist on his own Fire - Style jutsu. It would also be cool to see how Fairy Tail characters react to the ninja world's politics and strict hierarchies.
One idea could be that Naruto and his friends are transported to the Fairy Tail world. They could join forces with Natsu and the gang to fight a common enemy, like a powerful dark wizard who has the ability to control the Nine - Tailed Fox. Another plot could involve the sharing of magic and ninja techniques between the two groups. For example, Naruto teaching some Fairy Tail members basic ninja moves while they teach him some magic spells.
One interesting storyline could be where Naruto characters end up in the Fairy Tail world and vice versa, and they form a harem. For example, Naruto might meet Lucy from Fairy Tail and they start an adventure together, gradually attracting other characters into their group that develops into a harem. This could involve them facing common enemies from both universes.
Perhaps a plot where a powerful villain threatens both the Naruto and Fairy Tail universes. The characters then combine their skills, like Naruto's ninjutsu and Fairy Tail's magic. In the harem part, it could be that the main character, say a new character created for this fanfiction, has to navigate the different personalities and affections of characters from both series while they all work towards saving their worlds.
Perhaps the story could involve a powerful entity from the Naruto universe arriving in the Fairy Tail world. This entity, with godlike powers, is on a quest. Along the way, he meets different Fairy Tail characters. As he helps them with their struggles against dark guilds or powerful enemies, he earns the affection of multiple female characters, creating a harem situation. The story could then explore how he manages these relationships while still staying true to his own goals.
One idea could be Romeo going on a magical adventure with his harem members to save the Fairy Tail guild from a new dark threat. They might use their combined magical abilities to overcome obstacles.