Another interesting concept is a crossover in which the characters' powers are swapped. So, for instance, Kanda from 'D.Gray - Man' could have a Zanpakuto like in 'Bleach', and Renji from 'Bleach' could have an Innocence. This would completely change their fighting styles and how they interact with the other characters, creating a really unique fanfiction.
Maybe there could be a story where the Soul Society in 'Bleach' discovers the existence of the Black Order from 'D.Gray - Man'. This could lead to a lot of misunderstandings at first, as the two groups have different ways of dealing with supernatural beings. But eventually, they realize they can learn from each other's methods and join forces to protect both their worlds.
One idea could be that the characters from 'Bleach' and 'D.Gray - Man' are brought together to fight a common enemy that is a fusion of Hollows and Akuma. For example, Ichigo and Allen could team up, with Ichigo's powerful Zangetsu and Allen's Innocence combining their powers in unique ways to defeat this new threat.
A crossover with Fullmetal Alchemist could also be fascinating. Picture Edward Elric and the D Gray - Man characters collaborating to understand the mysteries of the Gate and the Innocence. Maybe they find out that there are connections between the two in terms of the source of power and the balance of the world. They could also face off against enemies that are a combination of Homunculi and Akuma.
One idea could be to have characters from 'D Gray - Man' interact with characters from another series in a post - apocalyptic setting. For example, they could team up to fight against a new type of Akuma that has emerged due to the crossover's unique circumstances.
One idea could be that the Akatsuki from Naruto are somehow connected to the Noah family in D Gray - Man. Maybe they are both after a powerful and ancient artifact that has the power to change both worlds. Another plot could involve Naruto characters having to deal with Innocence and Akuma in the D Gray - Man universe. For example, Naruto's chakra could be seen as a similar power source to Innocence and he has to learn how to use it to fight the Akuma.
One idea could be that the characters from Blue Exorcist are transported to the world of D Gray - Man. For example, Rin Okumura could meet Allen Walker. They might team up to fight a common enemy that is a mix of the demons from Blue Exorcist and the Akuma from D Gray - Man. Another plot could involve the exorcism techniques from both worlds being combined to defeat a super - powerful evil force.
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.
One idea could be the exorcists from D Gray - Man ending up in the world of Highschool DxD. They might initially be mistaken for devils or angels due to their strange powers. For example, Allen Walker's cursed eye could be seen as a very unique and perhaps threatening power. Another plot idea could involve the search for the Innocence in the Highschool DxD world. Since both worlds have different forms of power sources, it would be interesting to see how the Innocence interacts with the Sacred Gears. Maybe a character with a Sacred Gear could also be a potential host for the Innocence, creating a whole new dynamic.
One idea could be that the Exorcists from D Gray - Man end up in the Fairy Tail world due to a strange magical anomaly. They team up with the Fairy Tail wizards to fight against a common enemy that is causing chaos in both their worlds. For example, a powerful dark mage who has somehow acquired Innocence and is using it in a corrupted way.
Another interesting angle is to focus on the characters' relationships outside of the main battles. For example, how Allen Walker and Lavi interact during their down - time at the Order. You could create scenarios where they share stories from their pasts and bond in new ways.
One idea could be a story where the D Gray - Man characters attend a modern - day high school. Allen could be the new kid who seems a bit mysterious, and his strange weapon and abilities start to surface during gym class or self - defense lessons. Another idea is a school play where the characters are cast in roles that parallel their real identities in the D Gray - Man world, and things start to go wrong when real Akuma show up during the performance. A third idea could be a school council story where the characters have to deal with strange events happening around the school that are actually related to the Millennium Earl's plans.
First, you need to establish a strong reason for the two universes to collide. It could be something like a powerful, ancient evil that has the ability to disrupt both the world of Hollows and Akuma. Then, introduce the characters gradually. Let them interact in ways that are both true to their original personalities and also show growth as they face new challenges together. For example, Rukia from 'Bleach' might initially be skeptical of the Black Order's methods, but as she sees how they fight Akuma, she begins to respect them.