I'm not sure of a specific fanfiction on this, but you could start by looking on popular fanfiction websites like Archive of Our Own. Search for 'Zoya and Nikolai pregnant' in the search bar, and you might find some great stories.
One plot idea could be the challenges they face in preparing for the baby. For example, dealing with political unrest in their kingdom while also getting ready for the new arrival. Maybe there are those who oppose their relationship and see the pregnancy as a threat to their power or status.
Well, a fanfiction about Nikolai and Zoya's kiss might explore their relationship in-depth. It could be set in a specific fictional world. Maybe it shows their growing affection through this kiss, and how it changes their dynamic.
One popular theme is their relationship development. Fans like to explore how their feelings for each other grow and change over time, from initial attraction to deeper love.
There's a fanfiction called 'Zoya's Choice' which delves into Zoya's internal struggle between her loyalty to Ravka and her growing feelings for Nikolai. It's a really interesting take on their relationship as it's told from Zoya's perspective.
You can find Zoya and Nikolai fanfiction on Archive of Our Own (AO3). It has a large collection of fan - created works for various fandoms, including the world where Zoya and Nikolai exist.
Another option is to check out some niche forums or communities dedicated to the fandom of which Nikolai and Zoya are part. Sometimes, fans share their own fanfictions in these more specialized groups, and you can discover some unique and well - written stories there.
Love and romance are very common themes. Since they are often a couple in the source material, fanfiction usually explores different aspects of their love story, like how they overcome obstacles to be together.