One possible plot could be that they face challenges in protecting their unborn child from enemies who might want to use the baby's powers for evil. Since both are Martians, their child would likely have unique and powerful abilities that could attract unwanted attention.
It could show how they become more protective of each other. With the pregnancy, they might be more cautious in their actions, both in and out of their superhero personas. For example, Martian Manhunter might be more vigilant in protecting Miss Martian from any potential threats.
For Naruto, he could start out being his usual confident self but then be a bit out of his depth in Ms Martian's world. As he spends time with her, he could become more understanding of different ways of life. Ms Martian, on the other hand, might be shy at first but grow bolder as she learns from Naruto's fearless attitude. She could also develop her powers further by learning from Naruto's training methods.
There is also the concern about the influence on society. Such stories can promote a wrong view of how to interact with the other or the unknown. Instead of promoting understanding, respect, and cooperation, it spreads a message that is based on sexual exploitation. In a world where we are trying to promote equality and respect for all beings, real or fictional, these types of stories are not in line with those values.
I'm not aware of any such stories in mainstream science fiction literature. Science fiction usually focuses on concepts like exploration, technology, and alien cultures in a more family - friendly or at least non - sexual way.
Friendship and teamwork are also common themes. Since they are from different universes, they have to work together. They learn to trust each other and overcome their differences. In many fanfics, they form a strong bond that helps them face various challenges, whether it's a powerful villain or a difficult situation in a strange new world.
A possible interesting plot point is Naruto having to adapt his chakra control in the Martian atmosphere. Since Mars has different conditions than Earth, his jutsu might work differently. Another could be Naruto encountering Martian life forms and having to communicate or fight them using his ninja skills.
In some fanfics, Miss Martian might teach Naruto about her alien culture and technology. This could be really fascinating as Naruto is always eager to learn new things. He could then use some of this knowledge to improve his own village. It's also interesting to see how the two characters from very different universes interact with each other's friends and allies.
Well, it's difficult to say whether there is such a fanfic. Fanfic writers come up with all sorts of creative ideas, but this particular combination of Martian Manhunter and M Gahn in a pregnancy - related story might not be very common. There are so many different fanfic universes and themes out there. It could be that someone has written it, but it might also be that no one has thought of it yet or it's just not popular enough to be easily found.
The relationship could start with hostility. The human slave, fearing for his life and freedom, resists the Martian queen. But as the queen shows some kindness or unique understanding, the slave might start to trust her a little. Eventually, they could become friends or even more, like partners in some sort of inter - species cooperation.