The Akumas in D Gray - Man are created by the Millennium Earl. If Akumas were to enter the Black Butler world, they would be a major threat. Their ability to transform and their inhuman strength would be a challenge for the Black Butler characters. On the other hand, Sebastian's demonic powers and his ability to make contracts could potentially be used to manipulate or destroy the Akumas. The D Gray - Man exorcists are trained to fight Akumas specifically, so their skills would be very useful in this new setting. But the Black Butler characters' knowledge of the local environment and their own unique skills, like Mey - Rin's sharpshooting and Bardroy's combat experience, could also contribute to the fight against the Akumas in ways the D Gray - Man exorcists might not expect.
The characters from 'D Gray - Man' are used to dealing with the supernatural in the form of Akuma and the Noah. In a crossover with 'Black Butler', they would initially be confused by the different types of supernatural entities in the 'Black Butler' world. For example, Grell Sutcliff's unique brand of reaper insanity would be quite a shock to them. Meanwhile, the characters from 'Black Butler' would be equally baffled by the Exorcists' powers and the concept of Innocence. However, as they interact more, some might start to admire the skills and abilities of the others. For instance, Sebastian might respect Allen's combat skills, and Allen might be impressed by Sebastian's loyalty to Ciel, despite their different moral standings.
Rin Okumura's blue flames could be used in combination with Allen Walker's anti - Akuma weapon. For example, Rin could use his flames to weaken an Akuma while Allen strikes with his weapon for maximum damage. Shiemi Moriyama's ability to communicate with plants could be useful in a D Gray - Man setting. She could use plants to detect hidden Akuma or create barriers. The exorcists in D Gray - Man could learn from the Blue Exorcist characters about different ways to combat evil spirits.
One possible plot could be that the characters from 'D Gray - Man' are transported to the world of 'Black Butler'. Allen Walker and Sebastian could meet, and there could be a clash of their different moral compasses. Allen, with his mission to destroy Akuma, might initially view Sebastian's demonic nature as something to be eliminated. Meanwhile, Sebastian, who is bound to Ciel, might see Allen as a threat to his master's plans. This would lead to a series of intense battles and power struggles, with the other characters from both series getting involved in unexpected ways.
The characters would have some really interesting interactions. For example, Sebastian would likely be polite yet calculating when dealing with the Sohma family. He might see them as an interesting study, especially those with the animal curse. Tohru, being her kind self, would probably try to befriend Ciel and the others. However, Ciel might be initially suspicious of her overly - nice demeanor, as he's used to a world full of deception.
Kagome's spiritual arrows could be enhanced by Grell's death scythe in a strange way. Maybe their combined powers could create a new weapon or a powerful blast that could defeat a very strong foe. Also, Shippo's illusions could be used to deceive the enemies, and Bardroy's explosives could be set off at the right moment to create chaos among the enemy ranks, which would give Inuyasha and the Black Butler characters an advantage in battle.
Sebastian's superhuman strength, speed, and demonic powers would be a huge asset in Gotham. He could easily take on many of the thugs and villains. Batman, on the other hand, with his detective skills and gadgets, would be very intrigued by Sebastian. Alfred's combat and medical skills might be similar to some of the skills in the Black Butler world. Ciel's intelligence and strategic thinking could also be useful in Gotham's complex criminal underworld.
One plot idea could be that the characters from D Gray - Man are sent to the world of Black Butler to investigate a series of strange disappearances that seem to be related to a powerful Akuma - like entity. The Black Butler characters, on the other hand, mistake the D Gray - Man group for some sort of new threat. This could lead to some interesting battles and misunderstandings before they figure out they need to work together.
Naruto's chakra - based abilities and the magic in Black Clover are quite different. But they could interact in cool ways. For instance, if Naruto were to use his Rasengan in the Black Clover world, the magic users might be surprised at the concentrated power. And Black Clover mages could potentially use their magic to enhance Naruto's attacks or vice versa.
Another theme is the relationship between characters. Whether it's the complex relationship between the Exorcists in D Gray - Man or the master - servant dynamic in Black Butler, fanfictions love to play around with these relationships. They might create romantic pairings that are not in the original series or explore the deeper bonds between characters in different situations. For example, a fanfiction could explore what would happen if a character from D Gray - Man had to serve as a butler in the Black Butler world. This would completely change the power dynamics and create a whole new set of interactions.
The relationship between Naruto and Lavi would be quite fun. Lavi is a bit of a joker and Naruto's positive attitude would match well with him. They could team up and get into all sorts of adventures. Hinata might be a bit shy around some of the D Gray - Man characters at first, but she would gradually open up. The relationship between the Akatsuki and the Akuma would be very different. The Akatsuki are more about power and their own goals, while the Akuma are mindless servants of the Millennium Earl. However, some members of the Akatsuki might see the potential in using Akuma for their own ends, which would create a very dangerous situation for both worlds.