First, you need to do your research on both universes. Know the characters, their powers, and the settings well. For example, know all about Naruto's jutsus and Sebastian's demonic abilities. Then, think of a good plot device to bring the two worlds together. It could be a portal or a powerful spell. After that, start with an interesting introduction that hooks the reader, like Naruto suddenly finding himself in the Black Butler's Victorian - era London.
Power dynamics is another theme. Naruto is a very powerful shinobi and in the Black Butler world, there are different power hierarchies. Fanfictions often play with how Naruto's power fits in or disrupts the existing power structures in the Black Butler universe. For example, how he might deal with demons or the Queen's orders. And also, friendship is a theme. Characters from both series form unexpected friendships, like Naruto befriending Sebastian. Their different personalities and goals make these friendships interesting to read about.
It could be really interesting. Sebastian from Black Butler might find Naruto's ninja world full of chaos and yet fascinating. He could use his butler skills to blend in, while Naruto might be intrigued by Sebastian's elegance and power. Maybe Sebastian would be tasked with protecting Naruto for some strange reason, like a contract made by the Hidden Leaf Village elders.
One idea could be that Ichigo Kurosaki from Bleach somehow gets transported to the Victorian era world of Black Butler. He could be mistaken for a demon or some kind of otherworldly being. Sebastian, being the ever - curious demon, might be intrigued by Ichigo's powers and his strong sense of justice. Another idea is that Ciel Phantomhive has to deal with Hollows suddenly appearing in his world. He enlists the help of the Soul Reapers from Bleach to combat this new threat. They could form an uneasy alliance where the Soul Reapers are not used to the complex politics of the Black Butler world, and Ciel has to figure out how to work with these super - powered beings.
One popular Naruto Black Butler crossover fanfiction is 'The Shinobi Butler'. It combines the ninja world of Naruto with the Victorian setting of Black Butler in an interesting way. The characters from both series interact in a new world filled with mystery and action.
One idea could be that Yuuri Katsuki, being a world - class figure skater, is invited to perform at an event in the Victorian - era setting of Black Butler. Sebastian could be tasked with making sure the event runs smoothly and he's both intrigued and somewhat amused by Yuuri's unique form of art. Ciel, on the other hand, might initially be dismissive but then becomes interested in the grace and athleticism Yuuri shows on the ice.
Power dynamics is also a common theme. Kaneki has his own set of powers as a ghoul, and the characters in Black Butler have their own abilities. Fanfictions might play around with who is more powerful in different situations or how they can use their powers together. Another theme could be the exploration of morality. Both series touch on moral grey areas, and in the crossovers, it can be interesting to see how the characters' moral compasses interact when they meet.
One great Black Butler and Harry Potter crossover fanfiction is 'A Butler in Hogwarts'. It has an interesting plot where Sebastian, from Black Butler, somehow ends up in the wizarding world of Harry Potter. He uses his skills to navigate the strange new place while also causing some chaos. Another good one could be 'The Dark Arts of a Demon Butler'. It combines the dark elements of both universes really well.
The unique feature is the combination of two very different universes. Naruto's world is full of ninjas, chakra, and big - scale battles, while the Black Butler world is more about Victorian - era mystery, demons, and the underworld. When combined in fanfiction, it creates a whole new set of scenarios.
Character growth is also a common theme. Characters from both series are put in new situations. Naruto might have to adapt to the strict social hierarchies in the Black Butler world, and characters from Black Butler might learn from Naruto's values of friendship and determination. This allows for a lot of character development and makes the fanfictions more engaging as we see familiar characters evolving in new ways.