A lot of Grell x OC fanfiction has the theme of acceptance. Grell has his own eccentricities and the OC has to accept him as he is. At the same time, Grell also has to accept the OC's unique qualities. And there's also the theme of sacrifice. Sometimes either Grell or the OC has to sacrifice something for their relationship.
Well, a prevalent theme is the exploration of Grell's complex personality. Fanfics often show how the OC interacts with his flamboyant, yet deeply emotional side. Redemption is also a theme sometimes. Maybe the OC helps Grell find a new purpose or come to terms with his past actions. And of course, there's the theme of the supernatural world of Black Butler. The OC gets to experience all the dark and mysterious elements of that world through Grell, like dealing with other reapers, demons, and the afterlife in general. This makes for really exciting and often very emotional stories.
Love and relationships are also common themes. It could be a slow - burn type of love story where they gradually come to understand and care for each other. Or it could be a more angsty story about unrequited love, with one of them pining for the other while facing various obstacles, like differences in their duties or social standings within the afterlife hierarchy.
In sebastian x grell fanfiction, a frequent theme is power dynamics. Sebastian is a powerful demon and Grell, though not as powerful in the same way, has his own kind of strength. How they interact in terms of power, who dominates and who submits at times, is often explored. Another theme is the idea of fate. Whether they are fated to be together or are constantly fighting against their fated roles in their own worlds.
One common theme could be the clash of two very different worlds. Naruto is from a ninja world full of action and honor, while Grell is from a more gothic and supernatural world. So, there might be themes about how they adapt to each other's worlds.
A lot of Grell x Sebastian fanfictions have the theme of redemption. Maybe Sebastian finding a way to be redeemed through his relationship with Grell or vice versa. Also, the theme of sacrifice can be found. For example, one of them might have to sacrifice something important for the sake of their relationship. And there's also the theme of mystery, as their relationship is full of unknowns and secrets.
Another theme is adventure. They might be sent on some sort of mission or journey together, and during this time, their relationship develops. It could be a quest related to the supernatural elements in the 'Black Butler' world, like dealing with other demons or solving mysteries about the afterlife.
Another theme could be friendship turned into something more. They might start as colleagues or acquaintances in the Kuroshtisuji world, and then the fanfiction could develop their relationship into a deeper, more intimate one. There might also be themes of self - discovery as they get to know each other better and how that affects their own identities within the story.
One common theme is the exploration of Grell's unique personality. His flamboyant and passionate nature often plays a big part in the relationship with the reader. For example, in many fics, his love for the dramatic might lead to some comical or intense situations with the reader.
One common theme is unrequited love. Given their complex relationship in the original story, fanfictions often explore the idea of one loving the other more deeply than is reciprocated.
One common theme is the exploration of their contrasting personalities. Undertsker's calm and collected nature against Grell's more flamboyant and passionate self. Another theme is the idea of finding common ground in their differences. They may seem like an odd pair, but fanfics often show how they can complement each other.