Perhaps a story where Marian Cross is framed for a crime within the Order, and Allen is the only one who believes in his innocence. Allen then sets out to prove Marian's innocence, facing many obstacles and enemies within the Order. This could lead to a lot of action and also some deep emotional moments between them as they face this difficult situation together.
Marian is often seen as a bit of a rogue within the Order. Allen is a dedicated exorcist. A fanfic could show how these different personalities clash when it comes to dealing with the Noah. Marian might be more inclined to use unorthodox methods to deal with them, which Allen might initially oppose. However, as they face more and more dangerous Noah encounters, Allen could come to see the value in Marian's approach, and Marian could start to respect Allen's unwavering dedication. This exploration of their personalities can be a central theme in a story, with their relationship evolving as they learn from each other.
One popular theme is the exploration of Allen's internal struggle. In the fanfiction, it often delves deeper into his complex emotions regarding his past, his role as an Exorcist, and his connection to the Fourteenth. Another theme could be the relationships he forms within the context of Cross. It might show how his interactions with other characters are shaped by Cross's influence. Also, the battle against the Akuma and the Noah in unique scenarios is a common theme, presenting new and exciting storylines that deviate from the original plot but still maintain the essence of the D Gray - Man world.
One plot idea could be that Allen and Lenalee go on a special mission together in a new, uncharted area. They face strange akuma and have to rely on each other's unique abilities more than ever. For example, Allen's cursed eye and Lenalee's dark boots play crucial roles in fighting off the akuma hordes.
Maybe the Earl hides Allen in a parallel dimension where time moves differently. Allen has to find a way to get back to his own world while also dealing with the strange rules and creatures in this new dimension. The Earl's motives for doing this could be to keep Allen away from the Order and use his power for his own evil plans later.
One common theme is the development of the relationship between Cross Marian and his apprentice. It could be about the growth of trust, or the apprentice learning from Cross Marian's experiences. Another theme is often the exploration of the D Gray - Man universe from their perspective, like dealing with the Akuma and the Black Order.
One interesting plot idea could be Allen and Kanda being forced to team up on a mission in a completely new and dangerous area. Maybe they have to deal with a powerful Akuma that has unique abilities they've never encountered before. This would force them to rely on each other's skills, with Allen's Innocence and Kanda's sword - fighting abilities working in tandem.
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.
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 could be a young exorcist - in - training who looks up to Allen and is brought into the fold by Cross. This character could add a new dynamic to the group, perhaps being a bit naive at first but growing through their experiences. Another could be a mysterious figure from Cross's past who has some connection to Allen's powers. They could bring new secrets and plot twists. And a third option could be a healer type of character who joins the team. Since the battles in D Gray - Man are so dangerous, having someone to patch up Allen and the others would be a great addition.
I really like 'The Unspoken Connection'. In this fanfic, the chemistry between Kanda and Allen is palpable. It starts with a simple mission they go on together and gradually builds up their relationship in a very believable way. It's a great read for fans of the pairing.