One idea could be that a character is brainwashed by the Millennium Earl to turn against the Black Order. For example, Allen could be made to believe that the Order is the real enemy, leading to some intense battles and internal conflicts within the group.
Perhaps a side character gets brainwashed to spy on the Order. They could be fed false memories about the Order's so - called 'evil deeds' and then try to sabotage their operations from the inside. This could add an element of mystery as the others try to figure out who the mole is.
A more complex idea would be that multiple characters are brainwashed simultaneously. They could form a new group under the Earl's influence, thinking they are on a mission to 'save' the world. This would force the remaining non - brainwashed characters to not only fight against the Earl's akuma but also try to rescue their friends from the brainwashing.
You could also create a fanfic where the reincarnation process is not perfect. The characters may come back with some of their powers missing or distorted. This could lead to a lot of internal conflict as they struggle to adapt to their new, less - than - perfect selves while also facing the external threats that come with the D Gray - Man universe. They might have to find new ways to fight against the Akuma and the Millennium Earl without relying on their full set of original abilities.
In this brainwashed Allen fanfic, it could also be that the brainwashing is part of a larger scheme. The Millennium Earl might be trying to use Allen as a weapon against the Black Order. Since Allen has the Innocence and his special powers, if he's under the Earl's control, he could cause a great deal of damage. The fanfic could then detail the battles that occur as a result, with the Black Order having to fight Allen while also trying to save him. They might have to find some long - lost knowledge or a special item to reverse the brainwashing process.
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.
Next, create a believable reason for the brainwashing. In the 'D.Gray - man' universe, it could be related to the Millennium Earl's power or some new dark artifact. You can also think about the process of the brainwashing, whether it's sudden or gradual. A slow - acting brainwashing might add more suspense as the character slowly changes.
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 interesting 'D Gray - Man Fanfic' story could be about the characters' backstories that were not fully explored in the original. For example, a fanfic might delve deeper into Allen Walker's life before he joined the Black Order. Another could be a what - if scenario, like what if certain characters had different powers. There are also fanfics that focus on the relationships between the characters, such as the friendship between Kanda and Allen, and develop it in unique ways.
One plot idea could be that the Earl is hiding Allen in a parallel dimension only he can access. Allen has to figure out a way to escape while the Earl tries to convince him to join his side. Another idea is that the Earl disguises Allen as one of his Akuma, and Allen has to maintain the ruse while secretly trying to undermine the Earl's plans from within. Maybe the Earl hides Allen's Innocence in a way that Allen has to go on a quest to find it again, all while being under the Earl's watchful eye.
One idea could be a story where Mana and the Earl have to team up against a common enemy that threatens their existence in ways they haven't faced before. This would force them to put aside their differences and use their combined powers.