I think Loki would be really interesting in this crossover. In the Avengers world, he is a mischievous and powerful character. In a crossover with Black Butler, he could interact with Sebastian in some really complex ways, either as an ally or an enemy. Their battles of wits would be fascinating to read about.
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.
Definitely Ciel. Ciel has a tragic past and a strong will. When put in the company of the Avengers, his reactions to their powers and their code of conduct would be very interesting. He could also offer his own unique perspective on justice and vengeance, which might contrast or complement the Avengers' views.
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.
I think the Flash would be. His super speed is so unique and it would be fascinating to see how he interacts with the Avengers.
I think Hulk would be the most interesting. He's all about brute strength and smashing things. Facing Godzilla, he'd be like a tiny but very angry force against a huge monster. It would be a battle of the titans in a sense, and seeing how Hulk reacts to Godzilla's size and power would be really exciting.
I think it would be Loki. In this crossover, Loki could use his trickery to manipulate the Twilight vampires. He might see them as a new tool to achieve his goals. He could pretend to be on their side and then turn on them when it suits him. His magic and cunning would add a whole new level of chaos to the story.
I think Loki would be really interesting. He's always the trickster in the Avengers world. In a crossover with Glee, he could use his magic to create some wild and wacky musical illusions. He might even try to take over the Glee club and turn it into his own little kingdom of chaos.
Batman would be the most interesting. He doesn't have superpowers like the Avengers, but he's still a formidable hero. His presence in their world would be like a wild card. He'd be constantly analyzing the Avengers and their enemies, and his dark, brooding nature would contrast with the more flashy and super - powered Avengers.