Well, in alien impregnation fanfic, a frequent theme is the clash of cultures. You've got the human culture and the completely alien one. When impregnation occurs, it can bring up all sorts of issues like how the two species will co - exist, especially when there's a new life form involved. Also, the theme of survival is there. The human may have to figure out how to survive in a world where she or he is carrying an alien offspring.
One common theme could be the relationship transformation. The idea of impregnation often changes the dynamic between Buffy and Spike in these fanfics.
Well, the idea of hope is a big theme. Despite the difficult situation of the alien blockade, the characters in the fanfics don't give up hope. They keep fighting and looking for solutions. Also, the theme of sacrifice is there. Some characters may sacrifice themselves for the greater good, for the sake of breaking the alien blockade or saving others.
One common theme is the exploration of the unknown. Since it involves an alien pregnancy, there's a lot of mystery about how the pregnancy will progress, what the alien offspring will be like, etc. Another theme is the isolation of the pregnant character. Often, they are alone in their situation, not fully understood by others.
One common theme is human survival. It's all about how humans find ways to stay alive during the alien invasion.
The theme of acceptance is also quite common. As the alien performs on the stage, they gradually gain acceptance from the human audience or other performers. It can be a story of breaking barriers and building relationships between different species in the context of the stage environment.
One common theme is transformation. The obvious one is the physical transformation during the alien metamorphosis, which can be both fascinating and terrifying. Another theme is survival. Characters often have to struggle to survive in the face of the unknown changes brought by the metamorphosis. And there's also the theme of discovery, as they discover new abilities or aspects of themselves during the process.
Another theme is survival. Being abducted is a dangerous situation, and the characters usually have to fight for their lives. They may face threats from the aliens or the harsh conditions of the alien spacecraft. In some fics, the humans have to outwit the aliens to survive.
One common theme is the exploration of differences. Since it involves an alien, there are differences in biology, culture, and values. For example, how an alien might show affection could be very different from a human. Another theme is acceptance. The characters usually have to accept each other's uniqueness to build an intimate relationship.
One popular example is the 'Alien' movie franchise. In these movies, the Xenomorph has a rather terrifying form of impregnating its victims, which adds a lot of horror elements. Another could be some episodes in the 'Doctor Who' series that touch on alien concepts related to this in a more science - fictiony way.
One such story could be about an alien encounter where they come in peace to share their advanced technology. For example, a group of friendly aliens land on Earth. They communicate with humans using a unique form of telepathy. They help humans solve environmental problems like cleaning up polluted oceans and reversing the effects of deforestation. There is no impregnation involved, just a positive interaction between two species.