One common theme is mentorship. Since Celestia is an important figure in the MLP world and Spike is a young dragon, stories might focus on her guiding him. Another theme could be friendship that develops into something more special over time.
A prevalent theme is mutual respect. Despite their differences in age, species, and status, Spike and Celestia respect each other in these stories. This respect often forms the basis of their relationship. There are also stories that focus on the cultural differences between a dragon (Spike) and a pony (Celestia). How they bridge these differences and grow closer is an interesting theme that is often explored in Spike x Celestia fanfiction.
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.
A common plot could be a quest. For example, they might go on a quest to find a magical lemon tree that has the power to heal the land. Another plot could be a friendship - building one. They start off not really understanding each other but through some lemon - related events, like making lemon tarts together, they become better friends.
One common theme is the exploration of their complex relationship. Given their history of rivalry and shared vampiric nature, many fanfictions dig deep into how they interact with and feel about each other.
Another theme is loyalty. Despite their differences and occasional rivalries, they are often shown to be loyal to each other in fanfictions. For example, in many stories, they will go to great lengths to protect one another.
The theme of acceptance is quite common. Spike has to accept the fact that he has a harem and the responsibilities that come with it. And the harem members have to accept each other as well. Another theme could be adventure. Spike and his harem often go on adventures together, facing new challenges and exploring new places. This adds an element of excitement to the fanfiction.
A theme that frequently shows up is the contrast between their species. Spike is a dragon and Rarity is a pony. This difference can lead to some really interesting cultural and physical differences being explored in the fanfiction. For example, how Spike's dragon nature might affect how he courts Rarity compared to how ponies typically do it. It also gives room for the exploration of acceptance, as they learn to accept each other's differences. Also, the theme of friendship is intertwined. Even if the relationship might be romantic in nature, their friendship is always at the core and is developed further through the story.
Another theme is growth. As Twilight grows in her magic and knowledge, Spike also grows in his own way, and fanfictions often show how they influence each other's growth. They learn from their mistakes together and become better versions of themselves.
Growth is another theme. Both characters are shown to grow as individuals and in their relationship. Spike might grow in his confidence or magical abilities, while Twilight might learn more about the value of different kinds of relationships, especially the one she has with Spike. It's often a journey of self - discovery for both of them.
Some fanfictions also explore the theme of self - discovery. Both Spike and Rainbow Dash might discover new things about themselves through their interactions. For example, Rainbow Dash might learn to be more patient, while Spike might find out that he has more courage than he thought. These themes add depth to their relationship in the fanfiction world.