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.
Maybe a crossover with a fantasy world. Gray Fox, with his advanced combat skills, could find himself in a world of magic and mythical creatures. He might have to learn to adapt his combat style and form alliances with wizards or elves to survive.
A crossover with a post - apocalyptic world could also be cool. Gray Fox could be a lone survivor in a desolate landscape, using his stealth and combat abilities to fend off mutants and other threats. He could also team up with other survivors to find a new place to call home.
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.
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 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 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.
Perhaps there could be a crossover of the chakra system in Naruto and the spiritual energy in the fox demon world. A fox demon tries to learn chakra control and combines it with their own powers. This would create a whole new set of jutsu - like abilities. For example, they could create a fire - based attack using their fox demon fire and chakra to make it even more powerful.
A really interesting fanfiction idea would be to set Dorian in modern times. How would he adapt to the modern world with all its technology and different moral values? He might use social media to flaunt his beauty, but also face new challenges. People might be more accepting of his strange situation, or perhaps they would be even more horrified. And how would he deal with modern art? His relationship with his portrait could take on a whole new dimension in the digital age.
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.