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.
Common themes may include the relationship development during the pregnancy. For example, how the relationship between bunnymund and the other relevant characters evolves as they face this new situation together. There could also be themes of protection, as the pregnant character might need extra protection. Also, themes of acceptance within the fictional world or community might be present as it's an unusual situation in the context of the story.
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 friendship. Since they are from different 'worlds' in the original lore, seeing them form a bond is a big part of many fanfics.
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.
In 'narusasu mpreg fanfic', a frequent theme is the emotional journey of the characters. Naruto, being his usually exuberant self, might have to adjust to the idea of Sasuke being pregnant and all the responsibilities that come with it. Sasuke, on the other hand, may struggle with his own identity as a ninja and now as someone who is going to be a parent. There could also be themes related to the power dynamics between them changing. Since Sasuke has always been a very powerful and somewhat aloof character, pregnancy could make him more vulnerable, and Naruto would have to step up in different ways. Additionally, the theme of how their friends and the rest of the ninja world react to this situation is also quite common.
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.
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 is the exploration of the relationship between Arthur and Merlin. The mpreg often serves as a catalyst to deepen their bond.
Another theme might be the reaction of the wizarding world to such an unusual situation. It can be about how society views their relationship and the pregnancy, and how Blaise and Harry navigate through the potential prejudices and challenges. In many fanfics, this can lead to some interesting plot developments as they try to protect their family and each other.
In Percy Weasley mpreg fanfics, a prevalent theme is the adaptation to the new physical and emotional state. Percy has to deal with the physical changes of pregnancy, which can be a whole new experience for him. Emotionally, he may struggle with feelings of fear, excitement, or confusion. Additionally, themes related to the future of the child and how it will fit into the wizarding world, considering Percy's own status and family background, are also quite common.