One possible theme could be family. As Allen is the earl's son, his relationship with his family, especially his father, would be a central aspect. Another theme might be duty. He has the duty to uphold his family's name and status. Also, identity could be a theme. Allen may be trying to figure out who he is beyond just being the earl's son.
The theme of inheritance is quite likely. Not just in terms of material inheritance but also the inheritance of values and traditions. Allen may struggle with whether to accept or reject the old - fashioned ways of his family. Another theme could be social class. His position as the earl's son places him in a particular social class, and the fanfic could explore how he navigates the differences between his class and others, and how he deals with the expectations and prejudices that come with it.
Well, it could be a really interesting fanfic. Maybe it's about Allen's journey as the earl's son. It might explore his relationship with his father, the earl. There could be adventures he goes on, or perhaps challenges he has to face because of his noble lineage. It could also involve his interactions with other characters in the fanfic's world, like servants, other nobles, or friends. Maybe he has to learn how to be a proper heir and deal with the expectations that come with being the earl's son.
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.
A theme of transformation is likely. The earl, as he goes through the process of redemption, will probably transform from a flawed character to a more virtuous one. This could involve changes in his beliefs, values, and behavior. Another possible theme is the price of sin. He may have to pay a high price, whether it's in terms of relationships, status, or personal well - being, in order to be redeemed.
Perhaps in the fanfiction, the Earl and Allen have a moment of understanding. The hug could be a sign of a truce, even if it's temporary. It could be that they both realize they have more in common than they thought, like their connection to the Innocence in some unexpected way. And this hug is a physical manifestation of that new - found understanding. However, it's also likely that the hug is a part of a more complex emotional battle between them. The Earl might be using the hug to taunt Allen, reminding him of his own inner turmoil and the difficult path he has to walk as an Exorcist.
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.
One possible theme could be the struggle between light and darkness. Given Naruto's connection to Erebus (darkness), he would likely have to grapple with his dark heritage while still trying to uphold the values of the Hidden Leaf, which are more associated with light. Another theme could be identity. Naruto would constantly be trying to figure out who he is in relation to his father Erebus and his own ninja identity.
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.
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.
Some fanfictions focus on the idea of redemption for the Millennium Earl through his interactions with Allen. In these stories, Allen might be the catalyst for the Earl to see the error of his ways. Maybe Allen's unwavering belief in the good of people starts to make the Earl question his own motives and actions, leading to a series of events where they both grow and change.