A plot could involve a character from Fairy Tail, like Natsu, accidentally stumbling into the world of Black Butler. He might mistake the Phantomhive manor for an enemy lair at first. Then, as he realizes they're also fighting against various evils, he teams up with them. The Fairy Tail magic could also be a source of fascination for the Black Butler characters. Maybe Gray's ice - make magic could be used to combat some of the hotter - than - normal situations in the Black Butler world, like when dealing with demons that thrive in heat.
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 mages from Fairy Tail end up in the Clover Kingdom. They might be mistaken for enemies at first, leading to some intense battles. Then, as they realize they are all mages with similar goals, they team up. For example, Natsu could team up with Asta to fight a common dark magic threat that is too powerful for either of them alone.
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.
One idea could be that the Fairy Tail guild members are transported to the RWBY world. They could team up with Team RWBY to fight a common enemy like a new type of Grimm that is resistant to normal Huntsmen and Huntresses' attacks but vulnerable to Fairy Tail's magic. For example, Natsu's fire magic could be really effective against ice - based Grimm.
One idea could be that Naruto characters end up in the Fairy Tail world and have to participate in the Grand Magic Games. They could show off their ninja skills which would be a novelty there. Another could be a team - up where Fairy Tail mages and Naruto ninja join forces to fight a common enemy that threatens both their worlds, like an ancient evil that can travel between dimensions.
One idea could be that the characters from Danganronpa are transported to the Fairy Tail world. They could be forced to participate in a Grand Magic Games - like event where the losers face a 'punishment' similar to the Danganronpa executions. This would create a high - stakes and tense situation as the Danganronpa characters are not used to this kind of magic - based competition.
Well, a great plot for a Spyro and Fairy Tail crossover fanfic could involve Spyro's dragon powers being mistaken for a new type of lost magic in the Fairy Tail world. The Fairy Tail guild members might be intrigued and try to recruit Spyro. Meanwhile, Spyro is just trying to find his way back to his own realm. He could end up sharing some of his dragon skills with the mages, like his fire - breathing ability, which could be enhanced by Fairy Tail's magic, creating a whole new kind of power.