Maybe in this crossover, there's a dark magic at play that causes Yuuri to be transported into the world of Black Butler. Yuuri has to rely on his skating skills, which in this world have some unexpected magical properties. Sebastian might see Yuuri as an interesting anomaly and try to figure out how to use him to his advantage, while Ciel is suspicious of this strange newcomer who seems so out of place yet has a certain charm.
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.
A possible scenario is that there's a skating competition in the Black Butler universe and skaters from different dimensions are invited, including Yuuri. Yuuri's pure heart and determination are in contrast to the often dark and scheming nature of the characters in Black Butler. Sebastian could be drawn to Yuuri's positive energy and Ciel might be envious of Yuuri's ability to focus solely on his passion without the burden of his own dark past and responsibilities.
For Yuuri, he may become more assertive in the Black Butler world. He has to adapt to survive, so his shyness might take a backseat. Sebastian would still be his composed, devilish self, but he could develop a newfound respect for Yuuri's tenacity. Ciel, whose main goal is often revenge, might be distracted by Yuuri's presence. His personality could shift from being so single - minded to being more conflicted. Yuuri's focus on his art and self - improvement could make Ciel re - evaluate his own life's purpose, and he may start to wonder if there are other paths he could take besides revenge.
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.
In a crossover, the reapers from Black Butler could interact with Death in Supernatural. They might have different methods of collecting souls, and this could lead to some interesting conflicts or collaborations. For example, Grell Sutcliff could be overly excited to meet Death from Supernatural and try to show off his scythe skills, much to the annoyance of the more stoic Supernatural characters.
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.
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.
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.
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.
A fun plot could be a cross - over where the Trickster from Supernatural ends up in the Black Butler world. He starts causing chaos by playing tricks on the characters. Sebastian tries to outwit him while also protecting Ciel. Meanwhile, the Trickster's antics might expose some secrets within the Phantomhive household that were better left buried.
In 'yuri on ice crossover fanfiction', the opportunity to see familiar characters in new settings is really appealing. When it crosses with a fantasy series, like 'Fairy Tail', the figure skaters might find themselves in a world full of magic and quests. This changes their dynamics completely. Also, the fanfiction often explores how the skills of the figure skaters can be adapted or used in the new worlds. For example, in a crossover with 'Naruto', the skaters' balance and agility could be seen as valuable assets in the ninja world, which makes for some really creative story - telling.
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.