In 'A Darkling Sea' novel, the plot involves a team of humans encountering an alien civilization beneath the ocean. It's filled with suspense, as they navigate unknown territories and try to forge connections with the aliens while facing various obstacles and mysteries.
Mal could be a great addition. His relationship with Alina adds an interesting dynamic. He could be jealous of the Darkling's closeness to Alina and cause some tension. Also, Genya. Her skills as a Tailor and her own experiences in the Ravkan court can bring in more political intrigue and a different perspective on the relationship between Alina and the Darkling.
I would recommend 'Shadows of the Heart'. In this fanfiction, the story takes a different turn from the original. It explores what could have happened if Alina had made different choices regarding the Darkling. It's full of interesting plot twists and great character development.
One common theme is the power struggle. Alina has her own power, and the Darkling has his, and their relationship is often shaped by who has more power at different times. Another theme is redemption. Some fanfics explore whether the Darkling can be redeemed in Alina's eyes or in a more general sense.
You can find 'Darkling and Alina Fanfiction' on websites like Archive of Our Own (AO3). It has a vast collection of fanfictions for various fandoms, including this one. Just search for the relevant tags and you'll be presented with numerous options.
A plot could be centered around an alternate reality where Alina and the Darkling meet in a different context, like in a foreign land away from Ravka. They start as strangers, not knowing each other's identities, and gradually form a bond that is tested when their true selves are revealed.
One great 'alina darkling fanfic' is 'The Darkling's Redemption'. It delves into the complex relationship between Alina and the Darkling in a really interesting way. The author does a wonderful job of exploring their characters deeper than what we see in the original work.
The Apparat could be a main antagonist. He might see the baby as a tool to gain more power and control in Ravka. He could try to manipulate Alina into giving up the baby so he can use it for his own political and religious schemes.
Archive of Our Own (AO3) is a great place to start. It has a vast collection of fanfictions from various fandoms, including the Grishaverse where nikolai and the darkling are from. Just use the search function and type in relevant keywords like 'nikolai x the darkling' and see if any stories with the specific elements you want come up.