Sebastian's superhuman strength, speed, and demonic powers would be a huge asset in Gotham. He could easily take on many of the thugs and villains. Batman, on the other hand, with his detective skills and gadgets, would be very intrigued by Sebastian. Alfred's combat and medical skills might be similar to some of the skills in the Black Butler world. Ciel's intelligence and strategic thinking could also be useful in Gotham's complex criminal underworld.
A great plot idea is that the reapers from Black Butler somehow cross over to Gotham. They start interfering with the natural order there, which catches the attention of the Batfam. The Batfam initially mistakes them for some new, super - powered villains. Meanwhile, Sebastian is following the reapers to bring them back to their own realm and gets tangled up with the Batfam in the process. And what if Ciel was sent to Gotham on a mission by the Queen and he has to work with the Batfam to complete it.
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.
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.
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.
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.
Another aspect that makes it appealing is that it can explore different aspects of the existing Black Butler lore. For example, how would Snake's presence affect the power balance between Ciel and Sebastian? What new threats or opportunities would his presence bring? It gives fans of the original series a chance to see their favorite world from a fresh perspective, and it also attracts new readers who are interested in seeing how these two concepts blend together.