A typical plot theme is self - discovery. The OC human in the fanfic may start out not knowing much about themselves or their place in the world. Through their interactions with Celestia, they begin to understand their own strengths and weaknesses. Also, there could be a theme of power struggle. Maybe the OC human has some unique power that others in Celestia's realm either want to use or are afraid of, which leads to various conflicts and plot twists.
One common theme is friendship. The OC often forms a strong bond with Celestia, and they support each other through various difficulties.
When creating the OC human, it's important to think about their growth throughout the story. Start with them being naive about Celestia's world. As the story progresses, they should learn and change. For example, they might start out being afraid of magic but by the end, they master a certain type of magic. Their goals and desires should be clear. If they want to find a way back home to the human world at first but then start to fall in love with Celestia's world and want to stay and protect it, this shift in their goals can create an interesting and complex character arc.
One common theme could be mentorship. Princess Celestia, being a powerful and wise character, could mentor the human in magic or leadership skills.
One common theme is sisterly love and reconciliation. Since they are sisters in the original concept, many fanfics explore how they strengthen their bond.
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.
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.
A common theme is the cultural clash between the asari and human cultures. Since they are from different species, their ways of thinking, social norms, and values can be very different, and this often creates interesting conflicts and resolutions in the stories.
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.
Character development is key. In a good 'Human x Princess Celestia' fanfic, both characters should grow and change. For the human, they might start off as a naive outsider but become more confident and wise through their interactions with Celestia. Celestia, on the other hand, could become more open - minded towards the human world. For example, if the human challenges some of her long - held beliefs in a respectful way, it could lead to her personal growth.