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.
A plot could involve a contract between Ciel and one of the Hetalia characters. For example, England could make a deal with Ciel to help him with a particularly difficult case in exchange for something valuable. This could lead to a lot of comical misunderstandings as the two different universes collide. The Black Butler servants would have to adjust to having a 'country' as a guest or coworker, and the Hetalia characters would have to deal with the strict and often dangerous environment of the Phantomhive estate.
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.
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.
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.
One more interesting pairing could be Undertaker and America. Undertaker's mysterious and somewhat macabre nature against America's bold and brash attitude. America might be curious about Undertaker's strange hobbies and Undertaker could be intrigued by America's naivete and strong sense of justice. It would be fascinating to see how they interact, whether they become friends or end up at odds with each other. Also, Mey - Rin and Japan could have an interesting dynamic. Mey - Rin's clumsy but kind - hearted nature and Japan's reserved and traditional ways could lead to a unique relationship where they learn from each other.
Perhaps there could be a plot where the two main characters, Ciel and Maka, swap their partners. So Maka has to deal with Sebastian's sly and demonic ways while trying to achieve her goals in the Black Butler world. And Ciel has to work with Soul, a weapon with his own personality. This would force both characters to adapt to new situations and learn from each other's worlds.
One idea could be that the characters from Diabolik Lovers end up in the Black Butler universe and are mistaken for demons by the characters there. They then have to deal with Sebastian and Ciel while trying to find a way back to their own world. Another plot could involve a character from Black Butler getting trapped in the Diabolik Lovers' world and having to navigate the complex relationships among the vampire boys.
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.
One idea could be that Ciel and his servants from Black Butler end up in the Fairy Tail world due to a magical accident. Ciel might initially be suspicious of the Fairy Tail guild but then start to see the value in their sense of camaraderie. Another could be that a powerful dark magic from the Black Butler universe is threatening the Fairy Tail world, and the Fairy Tail mages have to team up with Ciel's group to stop it. For example, Sebastian could use his demonic powers in ways that both amaze and terrify the Fairy Tail members.
One idea could be that the countries from Hetalia are somehow transported into the Naruto world. For example, Italy could end up in Konoha and his carefree attitude might amuse or annoy the ninja there. Maybe he gets involved in a mission by accident and his unique skills from being a nation, like his knowledge of different cultures and languages, could come in handy. Another plot could involve Naruto being sent to the Hetalia world and having to deal with international relations in a very different way than he's used to in the ninja world. He might try to use his ninja way to promote peace between the nations there.