One common theme is discovery. Often, the OC is new to the world of Bunnymund and has to discover how things work, like the magic and the rules of the Easter realm. For example, in many stories, the OC will be amazed at how Bunnymund can manipulate time and space for Easter egg hunts.
One common theme could be recovery. After getting hurt, the story might focus on bunnymund's journey to heal, both physically and emotionally.
The theme of protecting each other also pops up a lot. Given their roles as Guardians, they are both in the business of protecting things (Tooth - teeth, Bunnymund - eggs). Fanfictions often show them protecting each other not just from physical threats, but also from emotional harm, which is a really sweet and endearing aspect of these stories.
I'm not an expert on this, but often there could be themes related to the challenges of the unexpected pregnancy, like how the character deals with the physical and emotional changes. Another theme might be the reactions of the other characters in the story towards the mpreg situation.
Often, there are themes related to sacrifice. Bunnymund might get hurt while trying to protect someone or something important. And also, the theme of isolation can be present, where his being hurt leads to him being alone and having to deal with the pain on his own.
One common theme could be the exploration of their different personalities. Jack Frost is often seen as more carefree and mischievous, while Bunnymund is more grounded. Their relationship in the fanfiction might play on how they balance each other out.
Another theme is the balance of nature. Since they are associated with different seasons, many fanfics explore how their relationship affects the natural order. For example, they might have to work together to ensure that winter doesn't overstay and disrupt the coming of spring.
A common theme is the interaction between Bunnymund and humans. It often shows how he copes with human emotions and behaviors that are so different from his own world.
One common theme is friendship. Since they are from different 'worlds' in the original lore, seeing them form a bond is a big part of many fanfics.
The fanfiction is likely captivating because it gives readers a chance to explore a different side of the world of the guardians. With Bunnymund being the focus, we already know he has a strong connection to nature and the seasons. The OC can bring in new relationships and conflicts. For example, they might come from a different part of the magical realm and have different ideas about how to protect the summer. This clash of ideas can create a lot of drama and tension, which makes for an exciting read.
A young summer faerie would make an excellent OC. Faeries are often associated with magic and nature. A young faerie could be curious about Bunnymund's role as a guardian. They could have the ability to manipulate small elements of summer, like making flowers bloom instantly or creating a gentle summer breeze. Their innocence and playfulness could contrast with Bunnymund's more serious and protective nature, creating a really engaging relationship in the fanfiction.