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 possible scenario in this crossover fanfiction could be the characters from D Gray - Man having to adapt to the unique supernatural environment of High School DxD. For example, Allen Walker might find himself in the middle of a battle between devils and angels in the High School DxD world. His Innocence could be perceived in different ways by the various factions there.
Allen Walker and Issei Hyoudou would be an interesting pairing. Allen has his serious and often brooding nature due to his past as an exorcist, while Issei is more of a pervy and hot - blooded character. Their interactions would be full of contrasts. Another good pairing could be Kanda Yuu and Rias Gremory. Kanda's cold and aloof personality would clash with Rias' more diplomatic and kind - hearted nature. It would be fascinating to see how they deal with each other.
In this crossover fanfiction, the plot could revolve around a power - stealing entity. It targets both the Innocence in the D Gray - Man world and the sacred gears in High School DxD. Characters from both universes must combine their forces to stop it. The D Gray - Man characters bring their knowledge of fighting the Akuma, and the High School DxD characters contribute their knowledge of the supernatural politics and different power levels in their world. This would lead to a lot of action - packed scenes as well as character development as they learn to trust and rely on each other.
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.
Well, it's the novelty. Combining Allen from D.Gray - man with other universes or stories creates new and unexpected situations. It allows fans to see their favorite character in different lights.
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 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.
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.
One idea could be that the characters from Highschool DxD end up in the Hellsing universe. Maybe Issei and his friends are mistaken for new types of vampires due to their devil powers. They then have to team up with the Hellsing organization to clear their names and also fight against a common enemy that is threatening both the devil and human worlds in this combined universe.