Perhaps the Nine - Tailed Fox inside Naruto is actually related to an ancient Akuma. This connection could draw the attention of the Millennium Earl from D Gray - Man. The Earl might see Naruto as a key to creating a new, more powerful type of Akuma. Meanwhile, the exorcists from D Gray - Man could view Naruto as both a threat and a potential ally. They would need to decide whether to help Naruto control the Nine - Tailed Fox or try to seal it away completely, which would put them at odds with Naruto and his friends.
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.
A cool plot could be a crossover of the character abilities. Allen Walker from D Gray - Man could learn some of Naruto's shadow clone jutsu. This would be useful in battles against the Akuma as he could send out clones to distract them while he attacks. On the other hand, Sasuke could be interested in the power of the Innocence and try to find a way to incorporate it into his own power set. Maybe there's a prophecy in both worlds that only the combined powers of Naruto and D Gray - Man characters can stop an upcoming great evil.
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 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.
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.
One plot idea could be exploring the backstories of the Noah family members in more detail. For example, how they became part of the Noah family and what their lives were like before. Another could be a story where a member of the Noah family tries to break away from their dark nature and find redemption. Maybe they form an unlikely alliance with an exorcist.
One idea could be an OC who is a Noah with a unique power that allows them to manipulate time on a small scale. This could lead to interesting interactions with the main characters as they try to figure out the OC's motives and abilities.
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.