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.
To make the plot really engaging, think about the power dynamics. In 'Bleach', there are different levels of Shinigami power, and in 'D.Gray - Man', there are various types of Innocence. When these two power systems meet in a crossover, it can create exciting battles and conflicts. For example, if Aizen from 'Bleach' somehow gets involved in the crossover, his devious plans could be thwarted by the combined forces of the 'D.Gray - Man' characters. The characters would have to figure out how to use their powers together to overcome such a powerful adversary. This would involve a lot of strategy and character development as they learn to work as a team.
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.
Start with a character from an unexpected universe getting invited to the Tournament of Power. For instance, a character from a fantasy world where magic is the main power source. Then, as they enter, they face challenges not only in the battles but also in understanding the technology and different power systems in the Tournament of Power world.
First, study the existing Noahs in D Gray - Man. Notice their characteristics, powers, and personalities. Then, for your OC, give them a distinct power that still fits within the world's rules. For example, if all Noahs have some form of elemental control, your OC could have control over something like shadow or light.
First, you need to decide which other series or world you want to crossover with. Then, research both 'D Gray - Man Hallow' and the other one thoroughly. This includes understanding the characters, their powers, and the overall lore. After that, come up with a basic plot idea, like a common threat that forces the characters from both to interact.
Use vivid descriptions. Since D Gray - Man has a very distinct gothic and dark fantasy setting, describe the environments in detail. When writing about Mana and the Earl, describe their appearances in a way that adds to their character. For instance, when the Earl is in his more menacing form, describe the way his robes move and his eyes glow. This will draw the reader into the world of your fanfic and make it more engaging.
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.
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.
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.
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.
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.