Well, it depends on your definition of 'good'. Some might focus more on the emotional aspect of Rin being pregnant, like how Archer would react and support her. Others could be more action - packed with the situation of the pregnancy adding an extra layer of drama to their adventures. You really have to dig through and read a few to find ones that suit your taste.
In this type of fanfic, Archer might use his unique skills and abilities to save his family. Maybe he has his signature archery skills that he utilizes to fend off threats. For example, he could be facing a horde of magical creatures and his precise shots are the only thing standing between his family and danger.
Well, in such a fanfic, perhaps Archer sacrifices his arm while saving Arturia from a great danger. Maybe it could be a powerful enemy attack and Archer jumps in the way, taking the brunt of it on his arm to protect Arturia. This would be a very heroic and selfless act on Archer's part.
It's a very interesting plot point. Once Rin finds out, she might start to analyze all their previous interactions. She'll likely remember how Archer sometimes showed hints of Shirou's personality. It could also make her wonder about the future of their relationship, whether they can still work together as they did before.
Some fanfictions focus on a post - Grail War scenario for Rin and Archer. Maybe they decide to team up to prevent future Grail Wars. They would use their combined skills, Rin's magic and Archer's combat abilities, to stop the forces that try to start new wars. This type of story can show how their relationship has developed after the intense events of the Grail War.
It could be that Archer's loss of an arm makes him more determined. He might start to train harder with his remaining abilities, and in the process, discover new skills. This could also make him more respected among his comrades. Arturia, on the other hand, might start to see Archer in a new light, not just as a fellow fighter but as someone she deeply cares about on an emotional level.
There are indeed some. Fanfic writers often explore this concept in creative ways. For example, one fanfic could have Archer using his unique skills and knowledge to rescue Arturia from a dangerous magical situation. Another might show how their relationship changes after the save, perhaps leading to a deeper bond or a new understanding between them.
Archive of Our Own is a great place to start. It has a large collection of fanfictions from various fandoms, and you can easily search for 'Archer and Rin pregnant' to find relevant stories.
Well, if we think about it, in the fanfiction when Rin finds out Archer is Shirou, it might completely change the dynamic of their relationships. For example, their interactions which were previously based on different assumptions will now be colored by this new knowledge. Maybe Rin will start to see Archer in a new light, with all the memories and experiences she has had with Shirou. And this could either strengthen their bond or create more distance depending on how the author of the fanfiction chooses to write it.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including Fate series which Rin and Archer are from. The site has a good search function that allows you to filter by characters, so you can easily find Rin and Archer fanfictions.