The theme of sacrifice also pops up. Sebastian, being a demon, might have to sacrifice his demonic nature to some extent for Ciel, or Ciel might be willing to sacrifice his own goals or even his soul for Sebastian. This theme makes the relationship between them more intense and deep, and it's often explored in different ways in various ciel and sebastian yaoi fanfics.
Another theme is the power dynamic between them. Ciel is the master, but Sebastian has his own power. Fanfics play with this dynamic, sometimes showing Ciel trying to assert more control or Sebastian subtly influencing Ciel. For example, in some fanfics, Ciel may try to break free from Sebastian's influence while still relying on him. It makes for very interesting reads.
Well, a frequent theme is the exploration of their relationship. It could be the development from a simple master - servant relationship to something more profound. Redemption is also a theme sometimes, like Sebastian seeking some form of redemption through his service to Ciel. And the theme of the supernatural is always present, given Sebastian's demonic nature and the dark world they inhabit.
One common theme is the reversal of their usual roles. Normally, Sebastian takes care of Ciel, but in these fanfics, Ciel is the one in the caring role, which creates an interesting dynamic.
One common theme is the adaptation of their traditional master - servant relationship into a high school context. For example, Sebastian might still be very protective of Ciel, like in the original setting.
In these fanfics, a recurring theme is the power dynamic. With Sebastian being jealous, it can either strengthen or weaken the power he has over Ciel. If he becomes over - protective due to jealousy, Ciel might gain some power in the relationship as Sebastian's actions are more driven by emotion. Another theme is the idea of loyalty. Sebastian's jealousy could be seen as an extreme form of loyalty, not wanting anyone else to have what he considers his (in relation to Ciel). And of course, the theme of love, in a very complex and often unspoken way, is also quite common as the jealousy is often a sign of deeper feelings.
One common theme is the power dynamic. Ciel, as a human, has power over Sebastian in the sense of their contract, but Sebastian's demonic powers far exceed Ciel's. Another theme is loyalty and betrayal. There's always the underlying question of whether Sebastian will truly fulfill the contract or if he might find a way to betray Ciel. Also, the theme of revenge is prominent as Ciel is seeking revenge for his family's death and Sebastian is a key part of that journey.
A common theme is possessiveness. Since Ciel and Sebastian have a rather special bond, any threat to that bond can trigger possessiveness which is often expressed as jealousy. Another theme could be the fear of loss. Ciel, having lost so much already, might be terrified of losing Sebastian to someone else, and this fear can manifest as jealousy.
Good ciel sebastian yaoi fanfic often has well - developed characters. It should capture the essence of Ciel's and Sebastian's personalities from the original source. For example, Ciel's determination and Sebastian's mysterious allure.
One aspect to consider in this fanfic is the possessive nature of Sebastian. His possessiveness towards Ciel could stem from his loyalty as Ciel's butler in the original story. In the yaoi context, it might be intensified, showing a deeper emotional connection. For example, Sebastian might be overly protective of Ciel, not just in a physical sense but also in terms of Ciel's relationships with others. This could lead to interesting plot developments and character dynamics.
I really like 'The Crimson Contract'. It takes their relationship in a unique direction. It has a great plot that isn't just about the romantic aspect but also about the power dynamics between Sebastian and Ciel. It's full of interesting twists and turns.