A more complex fanfiction could involve a shared enemy between the two worlds. A dark force that threatens both the ninja world in Naruto and the underwater world in 'My Bride is a Mermaid'. Naruto and the mermaid characters would have to team up, combining their different skills and powers, like Naruto's Rasengan and the mermaid's water - based magic, to defeat this common foe.
One idea could be a crossover where Naruto somehow ends up in the world of 'My Bride is a Mermaid'. Maybe he's on a mission and a strange jutsu goes wrong, transporting him there. He could meet the mermaid characters and get involved in their underwater adventures while also trying to find a way back to his own world.
Another fanfiction concept could be that the love story from The Princess Bride is re - imagined in the Hogwarts setting. Buttercup could be a new student at Hogwarts and Westley could be a mysterious figure who is actually a powerful wizard in disguise. Their love would face challenges not only from the typical Hogwarts rivalries but also from the dark forces that lurk in the wizarding world.
One idea could be that Marinette (from Miraculous Ladybug) discovers a magical seashell that transports her to the underwater world of The Little Mermaid. There, she meets Ariel and they team up to fight an underwater villain who is stealing the magic of both the surface and the seabed. Maybe the villain is after the Miraculous and Ariel's voice for some dark purpose.
One idea could be that the Phantom, with his mastery over music, somehow finds himself in the underwater world of The Little Mermaid. He might be intrigued by the mermaid's unique voices and decides to create a grand underwater opera with them. The mermaid protagonist could be both fascinated and a bit scared of this strange, masked figure from the surface world.
One idea could be to have characters from Naruto end up in the world of Danmachi. For example, Naruto himself could find his way into the dungeon and his ninja skills would be put to the test against the dungeon monsters. His shadow clone jutsu could be used in interesting ways, like sending clones into different dungeon levels to scout or fight. Another idea is to have the concept of chakra introduced into the Danmachi world. This could change the way adventurers fight and level up. Maybe some of the Naruto characters could train the Danmachi adventurers in new combat techniques.
One idea could be a crossover where Naruto is somehow transported to the Star Wars universe and meets Padme. Maybe he uses his ninja skills to protect her from the Sith.
Another concept is that Naruto is sent to Middle - earth. There, he could use his ninja skills and chakra powers. He and Gandalf could team up to fight Sauron. Naruto's ability to make friends and inspire others would be really useful in uniting the different races of Middle - earth against the common enemy.
A more complex idea is that the Matrix is used to create a virtual training ground for the shinobi. They enter the Matrix to practice their skills in a more controlled environment. However, they soon realize that there are some dark secrets within the Matrix, and Naruto has to lead the way to uncover the truth and free those who are trapped inside.
Another possibility is that Naruto visits the Townsville, the city where the PPG live. He might be confused by the modern technology there compared to his own world. The PPG could teach him about it while he shows them some of his ninja skills like tree - climbing and water - walking.
A third idea could be a story where Naruto's clones are mistaken for some kind of superpower by the Thundermans. Max might be intrigued and try to figure out how to do it himself. Meanwhile, the Thundermans could teach Naruto about modern technology which he has never seen in his own world. This could lead to some really interesting and comical situations as they try to understand each other's worlds.
Another possibility is that Pokemon exist in the Naruto world. Some ninja could have Pokemon as partners instead of traditional ninja animals. For example, Sasuke could have a Dark - type Pokemon like Umbreon that would match his cool and mysterious personality. And the Pokemon battles could be incorporated into the ninja battles, adding a new layer of strategy.