What are some interesting plot ideas for Black Butler mate fanfiction?

Another plot could involve an OC who is a powerful magician from a foreign land. They come to England seeking a powerful artifact that Ciel also has his eyes on. This would lead to a series of battles and negotiations between the OC, Ciel, and Sebastian, with the OC having their own set of goals and motives that might not be entirely clear at first.

One idea could be exploring Pluto's past before he became a demon. Maybe he had a family or a purpose that was lost. Another could be a story where Pluto forms an unlikely friendship with one of the other characters in the Black Butler universe, like Mey - Rin. It could be interesting to see how their different natures interact.

A great plot could be that the triplets are given the task of protecting a special guest in the Black Butler universe. They have to deal with various threats while also trying to figure out the guest's true motives. They use their combined intelligence and physical abilities to keep the guest safe.

One idea could be creating a story where a new character arrives at the Phantomhive manor with a special power that Ciel is initially suspicious of but later realizes can be useful in his battles against his enemies. For example, the power to communicate with the dead, which could provide valuable information in their investigations.

A cool plot for a 'Black Butler' crossover fanfiction might be a crossover with 'Kuroshitsuji' and 'Death Note'. Light Yagami could somehow cross paths with Ciel and Sebastian. Light might see Ciel as a means to an end in his grand scheme of creating a new world order, while Sebastian would be intrigued by Light's Death Note and the power it holds. It would be a battle of wits between them, with Ciel caught in the middle, trying to figure out how to use this situation to his advantage.

One idea could be exploring their past before the events of the main story. Maybe a story about their early training or a secret mission they had to undertake together as kids.

Well, you could write a fanfiction where the Black Butler characters are in an alternate universe. For example, a steampunk world. Sebastian could be a mechanic instead of a butler, and Ciel could be a young inventor. Or perhaps a story where Grell becomes Ciel's butler for a while, which would be really chaotic and full of humor. And then there's the idea of a story where the Phantomhive Manor is haunted by a different kind of spirit, not just the usual demons and ghosts that are part of the original story.

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 Ciel from Black Butler somehow gets transported to the world of Inuyasha. He could end up teaming up with Inuyasha to fight a common enemy, like a powerful demon that threatens both their worlds. Ciel's intelligence and strategic mind would complement Inuyasha's brute strength.

Maybe the Black Butler characters are sent to the Hetalia world. Sebastian could be intrigued by the different cultures and politics of the countries. He might use his charm and skills to manipulate some of the smaller countries for his own or Ciel's gain. Grell could go wild with excitement over all the new 'pretties' he could potentially reap in this new world filled with so many different characters. And Ciel would be constantly trying to figure out how to get back to his own world while also dealing with the strange international relations in Hetalia.