First, give your OC a unique backstory. Maybe they were an outcast on their home planet, which gives them a different perspective. For example, if they were shunned for their scientific ideas, they might approach the xenomorph with more curiosity than fear. Second, give them a specific skill set. It could be hacking into alien technology or having a deep knowledge of ancient alien languages. This will help them interact with the xenomorph in interesting ways.
The 'mystery' trope is also prevalent. The xenomorph's origin or purpose in the fanfiction might be a mystery that the OC is trying to solve. It could be linked to some ancient alien prophecy or a scientific experiment gone wrong. And then there's the 'transformation' trope. The OC may start to take on some characteristics of the xenomorph, either physically or mentally, as a result of their interaction.
The idea of a male OC and a female Xenomorph in fanfiction allows for a lot of creativity. It could be set in a post - apocalyptic world where the Xenomorphs have taken over. The male OC might be part of a resistance group and the female Xenomorph could be an outlier among her kind. Their story could be about breaking boundaries and finding common ground in a world full of chaos and danger.
One possibility is that the male OC could be a space explorer who encounters the female Xenomorph on an alien planet. Their meeting could start a chain of events that changes both of their fates. For example, the male OC might try to communicate with the Xenomorph using his advanced technology, and in the process, they form an unlikely bond.
One interesting Xenomorph OC fanfiction could be about a Xenomorph that has some unique genetic mutation which gives it the ability to communicate telepathically with other species. This makes it an outcast among its own kind and it embarks on a journey to find its place in the universe.
Xenomorph x Human Female fanfiction might also focus on the power dynamics between the two. The Xenomorph is typically a very powerful and feared creature, while the human female may be seen as more vulnerable in a traditional sense. But in these stories, she could potentially find ways to hold her own or even gain an upper hand through her intelligence or unique connection with the Xenomorph.
First, study the existing Xenomorph lore. Know their biology, like how they reproduce and their physical capabilities. Then, think of a unique trait for your OC. Maybe it has a different coloration or a special ability that sets it apart. For example, it could have the ability to change its shape slightly to blend into its environment better.
It could start with fear and curiosity. The human is initially terrified of the xenomorph but as they observe it, they notice some unique behaviors. This piques their curiosity and they start to approach it cautiously.
One aspect of Xenomorph x human fanfiction is the horror element. The Xenomorph is a classic horror icon, and when paired with a human, it creates a lot of tension. The fanfictions can be really intense as they explore the fear that a human would experience in the presence of a Xenomorph.
Survival is a very common theme. Since both xenomorphs and predators are dangerous and in a constant battle for existence in their own ways, fanfictions often revolve around their struggle to survive against each other or against a common threat. For example, in many stories, they might be on a planet with some other deadly force and have to work together to get out alive.