The archer could be a long - lost member of the Fairy Tail guild from the past. Maybe he was a powerful mage who specialized in archery magic and was sealed away for some reason. Mira's summoning accidentally breaks the seal and brings him back.
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.
Perhaps Sakura summons Archer to protect her village from a horde of demons. Archer uses his archery skills and strategic mind to fend off the demons while Sakura supports him with her magic.
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.
Perhaps in the fanfiction, Mira summons the archer as part of a magical experiment gone wrong. The archer is initially hostile, thinking that Mira has trapped him. But as time passes, he realizes that Mira had no ill intentions. They then form an unlikely alliance to fix the mess created by the experiment and in the process, they develop a deep friendship that helps them face other challenges that come their way in the Fairy Tail world.
The plot could be centered around a mysterious quest. Arthuria summons Archer Emiya to help her find a powerful relic that has been lost for centuries. Along the way, they face various foes who also seek the relic. Emiya's ranged combat skills and Arthuria's leadership and swordsmanship complement each other, allowing them to progress through the challenges.
The plot could be centered around a mysterious magical anomaly that Artoria can't handle alone. So she summons Archer Emiya. They then embark on a journey to figure out the source of the anomaly. During this journey, they face many challenges that test their skills and their relationship. Eventually, they succeed in solving the mystery and in the process, become closer as comrades.
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.