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.
I'm not really into that specific type of fanfic so I can't recommend any directly. However, you could try looking on fanfiction websites like Archive of Our Own. They have a wide variety of fanfics and you might be able to find some bunnymund mpreg ones by using their search function.
There's also 'Bunnymund: Guardian of the Hidden Realm'. This fanfic focuses on Bunnymund's role as a guardian. It has a great plot that involves him protecting his realm from various threats, and the way it portrays his determination and strength is really engaging.
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.
The unique combination of characters. Bunnymund has his own charm and abilities, and Sophie brings her own qualities. Their interaction can create a lot of interesting storylines.
One popular bunnymund and sophie fanfic is 'The Adventure of Bunnymund and Sophie'. It has an interesting plot where they go on a journey through different magical realms together. The story is full of exciting encounters and character development.
One popular 'jack frost x bunnymund fanfic' is 'Frost and the Bunny'. It beautifully weaves their different personalities together. The story has a lot of heart - warming moments as they learn to understand each other better.
In general, such a concept is quite out of the ordinary for traditional family - friendly fiction. Fan - fics can vary greatly in their content and tone. But when it comes to this particular idea of 'bunnymund gets jack pregnant', it's not a common theme in all - ages appropriate literature. There may be some fan - fic writers who try to explore unique relationships and concepts in a more innocent way, but it's not the norm.