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 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.
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.
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 relationship can start with mutual curiosity. The male oc might be fascinated by the alien nature of the female xenomorph, and she might be intrigued by his different biology. They could communicate through body language and simple sounds at first. For example, the male oc shows respect by not making sudden movements, and the female xenomorph responds by not attacking him immediately. As time passes, they start to trust each other more. Maybe the male oc helps the female xenomorph when she is injured, which deepens their bond.
The pregnancy could be a plot device to introduce new and unknown aspects of the Xenomorph species. Maybe the unborn Xenomorphs have special abilities that are revealed as the pregnancy progresses. Fanfiction writers can use this to build suspense and mystery, keeping readers engaged as they wonder what will happen when the new Xenomorphs are born.
In Amanda Ripley and Xenomorph fanfiction, often she is portrayed as a strong and resourceful character. She might be shown using her wits to survive encounters with the Xenomorphs. For example, she could use her knowledge of the ship's layout to outmaneuver the creatures, just like in the Alien franchise where her family has had to deal with these deadly beings.