Undertaker could be a great addition. He could be a mysterious undertaker in the real world as well, but with a modern twist. Maybe he runs a very avant - garde funeral home that also has some sort of secret underground operation. His knowledge of the afterlife and his rather spooky nature would add an element of mystery to the fanfiction. And his interactions with Ciel and Sebastian would be full of intrigue, as he always has his own motives.
I think Grell would be really interesting. In the real world, Grell's flamboyant and over - the - top personality would create a lot of chaos. He could be a fashion designer or something similar, still with his love for the color red and his chainsaw. His interactions with Sebastian would be hilarious, as Sebastian would be constantly trying to keep him in line while also dealing with the real - world problems.
Sebastian would be really interesting. He's a demon, so seeing him interact with modern - day society would be full of comical and also rather dark moments. He'd be confused by some things like traffic laws but would quickly figure out how to use his powers to his advantage in a world full of new challenges.
Sebastian would be the most interesting. His demonic nature and his loyalty to Ciel would bring a whole new dynamic to the Avengers. He could easily outmaneuver most of the villains with his wits and powers.
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.
Ciel and Dean Winchester would be an interesting pairing. Ciel is a young, determined noble who is used to getting his way, while Dean is a tough hunter with a lot of experience. They could learn from each other's ways of dealing with difficult situations. Ciel could teach Dean some of his strategic thinking, and Dean could show Ciel how to handle more physical threats.
Ron Weasley and Grell Sutcliff. Ron's clumsy but good - hearted nature would be a sharp contrast to Grell's flamboyant and somewhat bloodthirsty personality. Grell could be constantly frustrated by Ron's lack of finesse, while Ron would be completely bewildered by Grell's behavior. Their interactions would add a touch of humor to the crossover fanfiction.
I think Ciel and Lucy would make an interesting pairing. Ciel is this dark, brooding, and calculating character, while Lucy is more of an emotional and warm - hearted person. Their interactions could be really fun to explore, like when Lucy tries to get Ciel to open up more. Another great pairing could be Sebastian and Erza. Sebastian is a powerful demon with his own agenda, and Erza is a strong and no - nonsense mage. They would probably have some intense battles of wills and skills.
Sebastian would be really interesting. His demonic nature and his ability to adapt to different situations in 'Black Butler' would make him stand out in a crossover. He could use his skills to either help or manipulate the characters in 'Tokyo Ghoul'. His loyalty to Ciel in his own series could be contrasted with his new relationships in the 'Tokyo Ghoul' world, and it would be fascinating to see how he uses his powers to deal with ghouls.
Sebastian and Malleus Draconia would be an interesting pairing. Sebastian is a demon with his own set of skills and motives, and Malleus is a powerful fae. Their interactions could be filled with power plays and attempts to outwit each other. They both have an air of mystery and superiority, which would make for some great scenes.
Ciel would be a very interesting character to cross over. He has a dark past and a complex personality. In the Beauty and the Beast world, he could be a catalyst for change. Maybe he could see himself in the Beast and try to help him in ways that the other characters in Beauty and the Beast couldn't. Ciel's experience with dealing with demons and the supernatural could also be used to figure out the mystery behind the Beast's curse. And his relationship with Sebastian would add another layer of complexity to the story.
The Winter Soldier could be an interesting character to introduce. He has a complex history with Black Widow. His presence could add more depth to the story as he might be forced to choose between his loyalty to Black Widow and his own conflicting motives. Also, his combat skills and his own history of being brainwashed could create some interesting parallels with Ultron's attempts to manipulate Black Widow.