The plot might also involve Allen uncovering the secrets behind his reversion to red. There could be some ancient prophecy or hidden knowledge that is related to his condition. As he delves deeper into this mystery, he meets new characters who either help or hinder his journey. And throughout all this, his own identity is in flux. He's no longer the same Allen they all knew, and he has to figure out who he is now and what his purpose is in this new form.
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.
Obviously, Allen would be one of the main characters since it's about his reversion to red. He's the center of this change and the one who will be experiencing all the consequences.
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.
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.
The plot might also be that the Earl's hug is a way to get Allen on his side. He could be trying to convince Allen that there is a different path than the one he's currently on, and this hug is the first step in that persuasion.
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.
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.
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.
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.
Perhaps Allen and Neah would have an intense verbal exchange. Allen might question Neah's intentions right away, given Neah's somewhat mysterious nature in the D Gray - Man series.
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.