Perhaps Allen Walker discovers he has some latent wizarding abilities in addition to his Innocence. He could attend Hogwarts and form an unlikely friendship with Harry Potter. Together, they could use their unique powers to combat a new dark force that is causing chaos in both the D Gray - Man universe and the wizarding world. For example, Voldemort could somehow be connected to the Millennium Earl and they have to figure out how to stop them both.
One idea could be that the Exorcists from D Gray - Man end up in the wizarding world of Harry Potter. They might team up with the Order of the Phoenix to fight against a new threat that combines the dark magic of the Death Eaters and the Akuma from D Gray - Man.
Another plot could involve the Death Eaters trying to obtain the power of the Innocence. The D Gray - Man characters have to protect it while also navigating the complex politics and magic of the wizarding world. Hermione could be intrigued by the scientific aspects of the Innocence and try to study it, which could lead to some interesting interactions between the two groups.
One interesting crossover could be the use of D Gray Man's anti - Akuma weapons in the world of Harry Potter. For example, the Innocence - powered weapons could be seen as a new form of magic against the dark forces in Hogwarts. Another could be characters from D Gray Man attending Hogwarts, like Allen Walker learning magic along with spells like Lumos while still having his Exorcist powers.
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.
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.
Lavi and Ron Weasley could be an amazing pairing. Lavi's laid - back yet observant nature would contrast well with Ron's more hot - headed and loyal personality. They could get into all sorts of adventures together, maybe even accidentally stumbling upon a hidden connection between the two worlds.
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.
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.
One idea could be a story where the D Gray - Man characters attend a modern - day high school. Allen could be the new kid who seems a bit mysterious, and his strange weapon and abilities start to surface during gym class or self - defense lessons. Another idea is a school play where the characters are cast in roles that parallel their real identities in the D Gray - Man world, and things start to go wrong when real Akuma show up during the performance. A third idea could be a school council story where the characters have to deal with strange events happening around the school that are actually related to the Millennium Earl's plans.
One idea could be the exorcists having to deal with Akuma during a Halloween - themed mission. Maybe the Akuma are disguised as typical Halloween creatures like vampires or werewolves, adding an extra layer of confusion. Another plot could involve Allen Walker stumbling upon a town where a dark magic related to Halloween is causing chaos, and he has to figure out how to stop it while also dealing with his own inner demons.
A great plot for this kind of fanfiction might be a tournament in the style of the Triwizard Tournament, but with D&D elements. Instead of just three schools competing, there could be different factions from across the wizarding world. Each task could be designed like a D&D encounter, with the participants having to roll dice (in a magical way, of course) to determine their success in challenges such as fighting a dragon or solving a complex magical riddle.