Another theme is growth. For example, Elizabeth might grow from a naive girl into a strong woman because of her relationship with Ciel. And Ciel may also change in terms of his emotions towards Elizabeth. In some fanfictions, the theme of sacrifice also appears. One of them may sacrifice something for the other's well - being.
Love and devotion are very common themes. Most fanfictions focus on how deeply Elizabeth loves Ciel and vice versa, despite all the difficulties they face.
The theme of adventure often comes up as well. They might go on adventures together, facing all sorts of dangers and enemies. This not only tests their relationship but also allows for character development. It gives the writers a chance to explore different scenarios for Ciel and Elizabeth outside of the original story's framework.
One common theme is love against all odds. Their different social standings and the situations Ciel is often in make their relationship a struggle, but love perseveres. Another is the theme of loyalty. Elizabeth's loyalty to Ciel and vice versa is often explored.
A less common but still interesting theme is role - reversal. Some fanfictions might have Elizabeth taking on a more active or dominant role, perhaps in protecting Ciel or being involved in the more dangerous aspects of his world. This can bring a fresh perspective to their relationship and the 'Black Butler' universe as a whole.
One common theme could be identity. Since there are two Ciels, the question of who is the 'true' Ciel might be explored. Another theme could be family, as they are brothers and their family history and relationships play a role.
In some ciel x lizzy fanfictions, there is a theme of self - discovery. As they interact more, they learn more about themselves. Ciel might discover a softer side of himself because of Lizzy, and Lizzy might find more strength in herself. Also, the theme of loyalty often comes up, as they prove their loyalty to each other in different situations in the stories.
One common theme is the idea of the OC being a source of change for Ciel. Since Ciel has a rather dark and set - in - his - ways personality, the OC often comes in and shakes things up, like bringing more light into his life.
A common theme is the love triangle. Ciel and Alois both vying for the reader's attention, which creates a lot of tension. Another theme is the contrast between Ciel's more reserved nature and Alois' more erratic behavior. The reader often has to navigate these differences.
Love and sacrifice are also very common. Their relationship is often portrayed as a deep and loving one, and they are willing to sacrifice for each other. For example, in some fanfictions, Booker will put himself in danger to protect Elizabeth. Also, the theme of discovery is there. They are constantly discovering new things about each other and the world around them, which is a big part of their story in fanfiction.
Love and romance can also be a theme. Some fanfiction might explore a romantic relationship between Diane and Elizabeth. This could involve the development of feelings, dealing with jealousy from other characters, and the exploration of how their relationship might work in different settings.
The theme of secrecy also pops up a lot. Given the nature of their relationship and the unusual situation of mpreg, they often have to keep it a secret from others. This secrecy adds a layer of tension and mystery to the story, as they try to navigate their lives without others finding out about this major change in their relationship.
Another theme is the exploration of their complex emotions. They both have had difficult pasts, and fanfiction often delves into how they deal with their feelings for each other. For example, how Ciel's cold exterior might crack in the face of Alois's more wild and passionate nature. Also, themes of redemption and acceptance can be found, as they learn to understand and accept each other's flaws in these stories. This can be really interesting as it shows a different side to their characters that may not be fully explored in the original source material.