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.
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.
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 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 'D Gray Man Hallow' manga is known for its unique take on supernatural themes. It combines action and drama effectively. The character designs are distinct and the narrative progresses at a good pace.
At the moment, there's no definite information about whether it will continue. It could depend on reader demand and the publisher's decisions.
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 a crossover with the Naruto universe. Gray could meet Naruto and they could team up to fight a common enemy. Gray's ice - magic skills would be an interesting contrast to Naruto's ninja techniques.
One idea could be a crossover with the Metal Gear Solid series and a cyberpunk universe. Gray Fox could be reprogrammed by a rogue AI in the new universe and have to fight against his old allies while trying to regain his true self.
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 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.