Sometimes, there could be a theme related to the challenges she faces during the pregnancy. This could include physical challenges, like dealing with the discomfort, or more complex challenges like how her role in the story's world is affected by her being pregnant. There may also be themes of protection, with other characters in the fic trying to ensure her and the unborn child's safety.
One common theme is mentorship. Celestia is often Twilight's mentor, so the fanfics explore how Celestia imparts knowledge and wisdom to Twilight. Another theme is friendship. Despite their different positions, they form a deep friendship which is often depicted. And power is also a theme, like how Twilight grows in power with Celestia's guidance.
One common theme is the idea of redemption. Discord has a past of causing chaos, and in many fanfics, his relationship with Celestia is part of his journey towards redemption. For example, Celestia may offer him a chance to prove himself and do good for Equestria.
One common theme is sisterly love and reconciliation. Since they are sisters in the original concept, many fanfics explore how they strengthen their bond.
One common theme is possessiveness. Since Celestia is a yandere, she will be extremely possessive of Twilight. Another theme could be the power imbalance. Celestia has more power and authority, and this can create a complex situation when combined with her yandere nature. Love in an extreme form is also a theme. It's not a normal, healthy love but rather an obsessive one that can drive the plot.
A theme of redemption can sometimes be present. Maybe one of them has made mistakes in the past, and through their relationship with the other, they find a way to make up for those mistakes. Or perhaps they are both trying to redeem themselves in some way, and their bond helps them in this journey. Additionally, a theme of mystery could be involved, where they are trying to solve a mystery together, which deepens their connection.
Love and relationships are a common theme. In a yuri harem setting in the context of Celestia, it often involves multiple female characters having romantic feelings towards the main female character.
One common theme is the power struggle. Given Celestia's role as a powerful ruler of light and Sombra's association with darkness and power, many fanfics explore how they deal with their different levels of power. For example, in some, they have to find a balance between their powers to achieve a common goal.
The theme of power dynamics is often seen in Discord x Celestia fanfic. Celestia is a powerful alicorn princess, and Discord has his chaotic magic. Fanfics play with how these two powers interact and balance each other. Also, the theme of forgiveness is common. Celestia has to forgive Discord for his past misdeeds, and fanfics explore the journey towards that forgiveness in different ways. Some fanfics also have a theme of discovery, where they discover new things about each other and themselves through their interactions.
Often, you'll find themes like legacy. Since Celestia has such a long and important history in Equestria, Twilight has to deal with the expectations that come with being her daughter. Also, friendship still plays a role as Twilight's friends react to this new situation, adding a layer of complexity to their relationships.
A common theme is sisterhood. Given their relationship as sisters, many fanfics explore the deep bond between them, including how they rely on each other and support one another in ruling Equestria.