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.
One common theme is friendship. Their relationship often starts as a simple acquaintance and develops into a deep and meaningful friendship. They support each other through various difficulties.
There are also some fanfiction communities on Tumblr. You can search for bunnymund x tooth fanfiction there. Often, these fics are shorter and might have some really creative takes on their relationship. For example, you might find some that are based on AU (Alternate Universe) concepts like a modern - day high school AU where they meet as students.
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.
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.
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.
Friendship is a common theme. Jack and Tooth often start as acquaintances and develop a deep friendship. They support each other through various difficulties.
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.