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.
One common theme is friendship. Despite the harem aspect, Spike often forms deep friendships with the characters in the harem. Another is growth. Spike usually grows as a person through his interactions with the harem members.
One common theme could be friendship among the characters in the harem. Another might be Spike's growth and development as he interacts with multiple characters in an anthro - style setting.
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.
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.
One common theme is adventure. Given Spike's role in Ponyville and Ember's in the Dragon Lands, they can go on all sorts of exciting adventures together. Another theme is friendship. Their relationship often starts as a friendship and then may develop further. And of course, the theme of overcoming differences, as they are from different species.
A common theme is the exploration of their forbidden love. Since Spike is a vampire and Buffy is a slayer, their relationship is taboo in the Buffyverse, and fanfictions often play on this aspect. Another theme is passion and desire, which is where the 'lemon' part comes in.
One common theme is friendship. Since MLP is big on friendship, fanfictions often explore how Thorax and Spike's relationship grows in terms of friendship. They might go on adventures together and learn about each other's cultures and ways of life.