One idea could be a story where Mana and the Earl have to team up against a common enemy that threatens their existence in ways they haven't faced before. This would force them to put aside their differences and use their combined powers.
There could be many interpretations in such a fanfiction. Maybe the Earl has some complex motives behind hugging Allen. It could be a moment of false kindness, where he's trying to manipulate Allen in some way, perhaps by making Allen lower his guard.
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.
Use vivid descriptions. Since D Gray - Man has a very distinct gothic and dark fantasy setting, describe the environments in detail. When writing about Mana and the Earl, describe their appearances in a way that adds to their character. For instance, when the Earl is in his more menacing form, describe the way his robes move and his eyes glow. This will draw the reader into the world of your fanfic and make it more engaging.
Well, popularity of OCs in 'D Gray - Man Earl and OC Fanfiction' is a subjective matter. But generally, OCs that are designed to be morally ambiguous are quite popular. They add an element of unpredictability to the story. For instance, an OC who sometimes helps the Exorcists and sometimes seems to side with the Earl. Also, OCs that bring a new cultural or ethnic element to the D Gray - Man world are often liked. Maybe an OC from a far - flung land with different beliefs and customs. And OCs that are able to bridge the gap between the different factions in the D Gray - Man story, like the Exorcists and the Noah family, can also become very popular.
One interesting concept could be the unique relationships that an original character (OC) can form with the existing characters in the D Gray - Man universe. For example, an OC might have a complex friendship or rivalry with the Earl. Another concept could be exploring new powers or abilities that the OC brings to the story, which might change the dynamics of the fight against the Akuma. Also, the fanfiction could delve into different interpretations of the Earl's motives and backstory through the perspective of the OC.
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.
For Allen, you can focus on his internal struggle. Since he's being hidden by the Earl, he might be conflicted between his hatred for the Earl and the need to survive. Show his determination to escape and continue his mission. For the Earl, explore his motives for hiding Allen. Is it just to toy with the Black Order, or does he have a deeper, more sinister plan? Develop his character by showing his different sides, like his moments of calm calculation and his fits of maniacal laughter.
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.