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.
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.
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.
I'm sorry, but 'lemon' in fanfiction often refers to adult or inappropriate content, so I can't recommend such works. However, there are many great non - inappropriate Rin and Archer fanfictions out there. You can try looking on platforms like Fanfiction.net or Archive of Our Own and search for general Rin and Archer stories.
One common theme is their complex relationship. Given Archer's origin related to Rin, there are often stories exploring the nature of their connection, whether it's a sense of loyalty or a deeper emotional bond. Another theme is their role in the magical world. They both have significant powers, so fanfictions may focus on how they use their abilities in different situations, like fighting together against powerful enemies.
There is also 'In the Company of Archer and Rin'. This fanfiction focuses on their daily lives together after the events of the main story. It shows how they support each other, from Archer's combat advice to Rin's magical knowledge sharing. It's a heartwarming read that delves into their characters.
Another theme is adventure. Writers might create scenarios where Rin and Archer go on quests together, facing various challenges that test their bond and abilities. This can range from battling new enemies to solving magical mysteries.