One common theme is the exploration of Archer's complex personality. The OC often serves as a catalyst for this exploration. Another theme is the power dynamic between Archer and the OC, especially if the OC is a mage or has some special ability. Also, there's often a theme of redemption or self - discovery for Archer through his interactions with the OC.
He could be a wild card. Maybe he has different motives from his original role. For example, he might be trying to change certain events that he regretted in his own timeline.
One popular one could be 'Archer's New Adventure in a Crossover World'. It often features Archer in unexpected situations in a crossover setting, with interesting character interactions.
One common theme is friendship. Despite their different origins and personalities, Rin and Saber often develop a deep friendship in these fanfics. They learn to rely on each other and support one another through difficult times.
Good fanfics in this regard usually have a great plot. It could be something that ties into the original 'Fate/stay night' universe, but with a new twist involving Rin and Shirou. For example, a new threat that only they can face together, which tests and strengthens their bond.
Well, I've read a few. 'Archer's New Bond' is an interesting one. It delves into how an original character, a mage with a unique magic ability, meets Archer and they end up having to work together to face a new threat in Fuyuki. Another good one is 'The Archer and the Outsider OC' which shows the OC being from a different dimension and Archer's reaction to this strange new person. These fanfictions really expand on the Archer character and his potential relationships with original creations.
In the fanfiction, Archer might develop through his relationships. If he falls in love with a character, it could change his cold and detached nature, making him more empathetic and changing his priorities in the war.
Study Archer's character deeply. His backstory, his motives, and his skills are all important. You can then find ways to connect these aspects to the elements of the crossover world. For instance, if you're crossing over with a post - apocalyptic world, how would Archer's survival skills and his sense of justice play out? Also, don't be afraid to put your own spin on his character while still staying true to his core traits.
One popular 'fate stay night rin x shirou fanfic' is 'A New Day'. It explores their relationship in a post - Holy Grail War scenario, with Rin and Shirou facing new challenges together.