The most obvious antagonist would be the Millennium Earl. He has the motive and the means to brainwash Allen. He wants to destroy the Black Order and Allen, with his Innocence, is a major threat to the Earl's plans. So, by brainwashing him, he can turn that threat into an advantage.
It could also be a rogue ex - member of the Black Order. Maybe someone who was cast out or felt wronged by the Order and now wants revenge. They know Allen's importance to the Order, so they brainwash him to make the Order suffer. This character could have some special abilities or knowledge that they gained during their time in the Order, which they now use to carry out their devious plan.
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.
In some fanfics, Allen might be depicted as dead, but it really depends on the specific plot and the author's creativity. Fanfic writers often take liberties with the original story, so there could be various interpretations.
Loki could be a likely villain. He has a history of mind - controlling people and causing chaos within the Avengers. He might see Hawkeye as a useful tool to disrupt the team again. His mischievous nature and magic powers would make it easy for him to brainwash Hawkeye without being detected at first.
Well, in the fanfic, Allen might have a very intense encounter with the 14th. Maybe there would be a battle of wills between them, as Allen tries to resist the influence of the 14th. It could also involve some revelations about Allen's past or the nature of his Innocence that are related to the 14th.
One possibility is that the fanfic focuses on the relationships Allen has in this new state. His friends might be confused or worried about him. For example, Lenalee could be trying to reach out to him, but Allen is pushing her away because he doesn't want to put her in danger. And the Order might be divided on how to handle Allen now that he's different. They could see him as a threat or as a potential weapon against the Millennium Earl.
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 the Earl discovers Allen when he is in a vulnerable state. The fanfic might explore how the Earl's presence affects Allen's development. It could also show Allen's internal struggle between the kindness the Earl might show him and his loyalty to the Black Order. It could be a story full of psychological drama as Allen tries to figure out the Earl's true intentions.
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.
There are some fanfics that focus on Allen Earl's growth in power. They might describe his training sessions in more detail, and how he overcomes his weaknesses. Maybe he discovers a new form of Innocence or a unique way to use his existing powers. These fanfics often add exciting new elements to his character development.
Isolation is also a theme. Allen is often alone in his struggles, being the host of the 14th Noah. Fanfics may depict his feelings of isolation from both the Exorcist side and the Noah side, and how he copes with being different from everyone else.