Often, there could be themes related to the challenges and emotions that come with an unexpected and unusual pregnancy. It might include Dib's insecurities, fears, and hopes, as well as Zim's role in supporting or not supporting Dib during this time. The fanfic may also play with the idea of how this pregnancy could potentially change the course of the overall story in the Invader Zim world.
Another theme could be the reactions of the other characters in the Invader Zim universe to this strange occurrence. For example, how Gaz or the other alien characters might respond to Dib being pregnant. It can also explore the power dynamics between Zim and Dib in this new context, as the situation is likely to change their typical interactions.
One common theme could be the exploration of the relationship between Zim and Dib in a very different and extreme situation. Since Dib is male in the original and the concept of male pregnancy is introduced, it might deal with how they both adjust to this new reality.
I'm sorry, but 'mpreg' (male pregnancy) involves some rather unusual and often not widely accepted concepts in fanfiction. However, you can try searching on specialized fanfiction websites like Archive of Our Own (AO3). They have a large collection of fanfics, and you might be able to find some Zim x Dib related ones there.
One common theme is the rivalry between Dib and Zim. It's always a central part of these fanfics as their battle of wits and attempts to outdo each other.
One common theme is the 'opposites attract' idea. Zim is so outlandish and Gaz is so reserved, and fanfics often play on that contrast. Another theme could be 'unlikely allies'. Since they are both sort of outsiders in their own ways, some fanfics have them team up against common foes.
A common theme could be friendship. Despite their differences, writers might explore how Gaz and Zim could form an unlikely bond. Another theme might be adventure, as they both have unique personalities that could lead to exciting escapades.
One common theme could be the clash of their different personalities. Professor Membrane is often seen as a scientific and somewhat oblivious figure, while Dib is very much into the paranormal. So, the contrast can lead to interesting interactions.
One common theme could be the exploration of an unlikely relationship. Given Zim's chaotic nature and the Tallest Red's position in the Irken hierarchy, their interactions can lead to stories about breaking social norms within the Irken world.
One common theme could be the relationship development between Roy and Ed in the context of the unexpected pregnancy. There might be themes of how they deal with the challenges and joys that come with it.
The theme of self - discovery also runs through these fanfics. Stiles, in particular, may go through a process of self - discovery as he experiences the physical and emotional changes that come with mpreg. He has to learn to adapt and understand himself in a whole new light, which in turn affects his relationship with Derek.
One common theme is the exploration of the relationship between Thor and Loki in a very different and often more intimate context due to the mpreg aspect. It might involve themes of protection, as Thor would likely be very protective of Loki during the pregnancy.
One common theme might be the exploration of their relationship in a new and different context. With the mpreg element, it could be about how they deal with the unexpected situation together.