Perhaps the OC is a survivor from a small, isolated community that has developed its own unique way of dealing with the Death Stranding. They have a different set of beliefs and customs compared to the larger settlements. When they meet Sam, there is a clash of cultures at first. But as they get to know each other, they learn from one another. The OC could introduce Sam to some of their community's survival techniques, like a special way of purifying water or a new type of shelter construction that is more resistant to timefall.
One idea could be that the OC is a courier with a special ability to sense BTs from a greater distance. This could lead to various adventures as they try to avoid danger while making deliveries. For example, they might have to take a detour through a particularly rough terrain to avoid a large group of BTs, and in the process, discover a hidden cache of supplies left by an old pre - apocalypse outpost.
Another character could be an old hermit who has lived in isolation for a long time. They have a deep understanding of the land and the strange phenomena that occur. The hermit could act as a sort of guide or mentor to the OC, sharing ancient wisdom about how to navigate the areas filled with BTs and timefall without getting too damaged.
An interesting OC fanfiction could be about a Death Note owner who doesn't want to use it for evil but for good in a very specific way, like only targeting truly heinous criminals who keep escaping justice. They could have moral dilemmas throughout the story.
The OC's Mystic Eyes of Death Perception might attract the attention of a powerful villain who wants to use their eyes for their own evil purposes. The OC then has to go on the run, while also trying to figure out a way to stop the villain from getting what they want. Along the way, they meet other characters who help them develop their power further.
One idea could be to create a Shinigami OC who is a bit of an outcast among the Shinigami. Maybe this OC doesn't like the idea of just writing names in the Death Note and wants to find a more 'humane' way to deal with humans, which could lead to all sorts of interesting conflicts with other Shinigami. For example, they might try to influence humans in a positive way before their time is up instead of just causing their death.
One idea could be exploring the backstory of the main character in the fanfiction. Maybe they had a near - death experience in their childhood that shaped who they are now.
Another plot could revolve around an OC who is a Spartan trainee that doesn't quite fit the mold. They have a different way of thinking and approaching combat. In Halo 4, they end up being the key to solving a mystery related to the Prometheans. Maybe they find a way to communicate with the Prometheans and convince some of them to stop fighting the humans.
One idea could be an Otsutsuki OC who comes to Earth not for conquest but to study the unique life forms here. They could befriend some of the local ninjas and learn about the concept of friendship, which is alien to the Otsutsuki. This would lead to a series of adventures where they have to protect their new friends from other more hostile Otsutsuki members.
One idea could be an OC who is a long - lost relative of Moana. This OC could have been separated from the family during a great storm years ago. They could have unique skills, like being able to communicate with a different type of sea creature that Moana has never encountered. This would bring new adventures as Moana and the OC try to figure out how this new discovery could help their island.
One idea could be exploring the origin of the OC in the covenant world. Maybe they have a special power that sets them apart from the regular covenant members.
One idea could be an OC who is a new Wolfblood in a pack and has to prove themselves. Maybe they have a unique power that the others don't have and they have to learn how to control it while fitting into the pack's hierarchy.