Perhaps there is an ancient artifact that has the power to change the fabric of both the Naruto and Harry Potter universes. Mr. Black discovers its existence and realizes that only Naruto and Harry Potter together can stop it from falling into the wrong hands. Along the way, they face many challenges. Naruto has to deal with the strange magic in the wizarding world, and Harry has to adapt to the ninja way of life. Their different cultures and powers clash at first, but they gradually learn to work together and form a strong bond in the process of protecting their respective worlds.
One idea could be that Mr. Black is actually a wizard who has some connection to the magical world of Harry Potter and he somehow ends up in the Naruto universe. Maybe he has a special mission there related to a powerful jutsu that has magical properties similar to those in the wizarding world. And Naruto, being Naruto, gets involved in this strange situation and they have to team up with Harry Potter to solve a problem that threatens both worlds.
One idea could be that Cassiopeia Black, with her pure - blood heritage knowledge, secretly aids Harry in understanding the deeper secrets of the wizarding world's noble families. This could involve her revealing hidden spells or traditions that give Harry an edge against Voldemort.
One interesting plot could be a secret mission within Hogwarts where a group of students form a black ops - like team to uncover a hidden threat that the teachers are unaware of. They might use magic in unconventional ways, like combining spells for stealth and detection.
Maybe some of the Hogwarts students are sent to the Naruto universe. They would be amazed by the ninja techniques. Hermione, for instance, would be eager to study how chakra works and might even find a way to combine it with magic. Harry could learn some ninja stealth skills which would be useful in his fight against Voldemort.
In this fanfiction, an exciting plot could be Naruto's encounter with the Dark Arts. Given his strong will and positive nature, he might have a unique way of combating dark wizards. He could use his sage mode to sense the presence of dark magic and his Rasengan could be transformed into a powerful anti - dark magic spell. His relationships with the characters from Naruto could also be mirrored in the Harry Potter world. For instance, he might see Ron as similar to Shikamaru, in terms of being a laid - back but reliable friend, and Hermione as a bit like Sakura, smart and strong - willed.
An interesting plot could be that there is a prophecy in both worlds that foretells the coming of a being with the combined powers of magic and chakra. Harry Potter and Naruto are both candidates in their respective worlds. Kurama, seeing the potential in their combined strength, decides to guide them both towards fulfilling the prophecy. Along the way, they face many challenges, like learning about each other's powers and cultures. Harry has to adapt to the physical and combat - oriented world of Naruto, while Naruto has to understand the more intellectual and magical aspects of Harry's world. And they have to deal with enemies who don't want the prophecy to be fulfilled.
One idea could be that Harry Potter is reborn in the Naruto world as an Uzumaki. He could use his magical knowledge in unique ways, like using magic to enhance his chakra control. Maybe he creates new jutsu that combine magic and chakra, such as a spell - enhanced Rasengan.
One plot idea could be that Fem Harry Potter, while on a magical mission, stumbles into the werewolf territory where Jacob Black is. They initially have a misunderstanding as Jacob is protective of his pack's land, but then they discover they both have a common enemy in the form of dark magic users who are trying to control werewolves for their evil plans. So they team up to fight against this threat.
Perhaps Sebastian Michaelis could be summoned by a Hogwarts student who is dabbling in dark magic. He appears in the wizarding world and causes chaos. Harry and his friends suspect something is wrong and start to investigate. Along the way, they discover the existence of demons in a different context than they thought, and they have to use both magic and their wits to deal with Sebastian and the problems he brings.
Another interesting plot could be that Harry's transformation into a Black Lion is a sign that he has a deeper connection to Salazar Slytherin than anyone thought. It turns out that Slytherin had an affinity for lions in a different way than Gryffindors. This discovery causes a stir in the wizarding world, and Harry has to deal with both the prejudice and the new opportunities that come with this revelation. He could use his newfound connection to Slytherin's legacy to bring together the houses in a way that has never been done before in the fight against the Dark Lord.
One idea could be that Harry discovers his true parentage and has to struggle with his identity. He might be torn between the side of good that he has known all his life and the dark legacy of Bellatrix. Another plot could involve him facing discrimination from both the wizarding world that hates Bellatrix and the Death Eaters who might see him as a tool or a threat. For example, he could try to use his knowledge of Bellatrix's magic, which might be more complex and dark - tinged, to fight against Voldemort in a different way than in the original story.