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.
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.
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.
Sorry, I haven't actually read this specific fanfic, so I can't give a definite summary. But generally, it should be about Allen's encounter with the 14th, which might include some kind of conflict or discovery.
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 possible aspect of an early fanfic where Allen meets Neah could be the exploration of their complex relationship. Maybe it delves into how Allen initially reacts to Neah's presence, whether with suspicion or a strange sense of familiarity.
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.
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.
The D Gray Man Allen manga is known for its depth. It explores complex themes and has a diverse cast of characters. The action scenes are exciting and the world-building is immersive.