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.
One common theme could be mentorship. Princess Celestia, being a powerful and wise character, could mentor the human in magic or leadership skills.
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 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.
One common theme could be mentorship. Celestia is an older and wiser character, so she might mentor Rainbow Dash in some way. Another theme could be friendship that develops into something more special over time.
One common theme could be friendship. Luna and Celestia are sisters in many interpretations, so fanfics might explore their sisterly bond, how they support each other through difficult times. Another theme could be power and responsibility. Since they both have important roles in their fictional world, stories could be about how they manage their powers and the responsibilities that come with them.
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.