The 'Uncharted' franchise has a large following of all ages, and sexual fanfiction would not be in line with the values that the creators likely intended to promote. It's better to focus on the positive and creative aspects of the 'Uncharted' world through appropriate fan - made content.
Writing or seeking out sexual fanfiction is inappropriate and often violates the terms of use of the original work and ethical standards. So, I cannot recommend any such stories.
First, think about their backstory. For example, if they're an adventurer, what led them to this life? Maybe they grew up in a family of treasure hunters. Second, give them unique skills or flaws. A skill could be expert marksmanship, and a flaw could be a fear of heights. This makes them more human. Finally, tie them into the existing Uncharted world. Maybe they have a past with one of the existing characters.
Perhaps an OC is a young archaeology student who idolizes Nathan Drake. They manage to get involved in one of his expeditions through sheer determination. Along the way, they learn from Drake and also teach him new things about modern archaeological research methods. It could be a story of growth and adventure for both characters.
One great 'uncharted fanfiction' story could be those that focus on the origin of the main characters. For example, a story that delves into Nathan Drake's early days of treasure hunting. Another could be a fanfiction where Chloe and Nathan team up in a new and unexpected adventure. There are also some that might explore the hidden secrets of the ancient artifacts in the Uncharted universe.
I think Uncharted Story is good. The way it unfolds keeps you hooked, and the settings and action scenes are well-done. But, of course, it's subjective - different people have different standards for what makes a story good.