One idea could be that Naruto is somehow transported to the Marvel universe and his unique abilities draw the attention of several female characters. They form a sort of harem around him as they are intrigued by his ninja skills and his never - give - up attitude. Another could be that a magical or scientific accident combines elements of the Naruto world and the Marvel world, creating a situation where a harem forms around Naruto as he navigates this new and strange combined reality.
A possible plot could be that Naruto is on a mission in a dimension that bridges the Naruto and Marvel worlds. There, he meets various female superheroes and villains. Some of the villains might initially want to use him for their own gain but end up being charmed by his positive nature. The female superheroes, on the other hand, see him as a potential ally and friend. As time goes on, this group of diverse female characters forms a harem around Naruto, leading to a lot of interesting interactions and adventures as they face threats that require both Naruto's ninja powers and the Marvel characters' superpowers.
Maybe the OC is a scientist who accidentally creates a device that links him to the minds of the female Marvel characters. This leads to a series of comical and heart - warming interactions as they get to know each other on a deeper level. For example, he might initially be confused by their different personalities but then starts to fall for them one by one.
In a 'harry potter and marvel harem fanfiction', it could be that a powerful magical object from the Potter universe is lost in the Marvel universe. A group of Marvel heroes and some Harry Potter characters team up to find it. During the adventure, the male Potter characters develop relationships with the female Marvel characters, resulting in a harem - centered plot.
One idea could be that Luffy accidentally stumbles into a parallel world full of female characters who are all smitten with him. Another could be that in a new island they visit, there are a group of girls who have unique powers and they all want Luffy to be their leader and partner. Maybe there's also a plot where a magic spell makes all the female pirates in the Grand Line be attracted to Luffy and they keep following his ship.
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.
One plot idea could be a tournament - style competition where characters from both Naruto and Rosario Vampire universes participate. The harem aspect could come in as different characters form alliances and friendships. Another idea is a mystery where they have to solve a strange phenomenon that is affecting both worlds. Maybe some characters are losing their powers and they have to figure out why.
A plot could be that the shinju has been transported to the Marvel universe and is causing chaos. The Avengers think it's a new kind of alien threat. Meanwhile, Naruto, who has somehow also ended up there, realizes it's the shinju. He has to convince the Avengers that he can handle it his way. But some of the Avengers don't trust him at first. So, there's a conflict between Naruto's way of dealing with the shinju and the Avengers' methods.
One plot idea could be Naruto being transported to the Marvel Universe during a strange space - time rift. He might team up with Spider - Man. Naruto's ninja skills like shadow clones and rasengan could be a great asset in battles against Marvel villains. Another idea is that the Infinity Stones somehow end up in the Naruto world, and the Avengers have to go there to retrieve them, leading to a clash and then an alliance between the two groups.
One idea could be that they team up to face a new super - villain who has the power to manipulate time. As they work together, they start to understand each other's strengths and weaknesses better. This could lead to a deep friendship and mutual respect between them.
Perhaps Naruto is on a mission near the sea and gets caught in a magical current that transports him to a hidden mermaid kingdom. In this kingdom, he is seen as a special being. The mermaids, being a matriarchal society, have different ranks of mermaids vying for his attention. He has to figure out how to get back home while also handling the mermaid harem drama.
One idea could be Naruto forming a unique bond with each member of the harem in different ways. For example, with Konan, he might discover her love for art and they could bond over creating origami together. Another could be Naruto teaching his harem members some of his ninja techniques, which would lead to interesting training sessions and character development.