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 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.
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.
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.
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.
There could be. 'D.Gray - Man' has a large and dedicated fan base, and fanfiction writers often like to take the story in unexpected directions. The idea of Allen dying is a rather dark and dramatic twist that some writers might be interested in exploring. You can start your search on popular fanfiction websites like Archive of Our Own or FanFiction.net.
Yes, there are many. Some fanfictions might explore Allen's backstory more deeply, like his time before joining the Black Order. Others could focus on his relationships with other characters, such as his bond with Lavi or Kanda.
One possible scenario could be that Allen is forced into a situation where he has no other choice but to become a Noah. Maybe he is tricked by a powerful Noah who manipulates his past traumas and the power of Innocence within him. The dark side of the Noah's power might seem appealing in some way, like offering him a way to end his constant struggle against the Akuma and the Earl.