Yes, there are likely fanfictions out there with such a plot. The 'Attack on Titan' fandom is quite large and creative, and many fans like to explore different relationship scenarios for the characters, including Eren and Mikasa having a family. However, the quality and nature of these fanfictions can vary greatly.
Archive of Our Own (AO3) is a great place to start. It has a large collection of 'Attack on Titan' fanfictions, and you can use the search function to look for stories specifically about Eren and Mikasa being pregnant. Just type in relevant keywords like 'Eren Mikasa pregnant' and browse through the results.
One plot idea could be that she is kidnapped by a group who wants to use her as a bargaining chip against the Survey Corps. Maybe they demand the release of their comrades in exchange for her safe return. Another could be that she is kidnapped by a mysterious figure who has a vendetta against her family and wants to extract information from her about her past. Or perhaps she is kidnapped by a cult that believes she has some special power and they want to use it for their own dark purposes.
The theme of family and legacy might also be prominent. Eren and Mikasa's relationship has always been complex, and with a pregnancy, it adds a new dimension to their family dynamic. Fanfics might explore how they plan to raise their child, what values they want to pass on, and how their family will contribute to the future of the world they live in. This can be especially interesting considering the post - apocalyptic setting of 'Attack on Titan'.
I'm sorry, I can't directly recommend specific 'Mikasa Eren' fanfics where Mikasa is pregnant. However, you can try searching on some well - known fanfic platforms like Archive of Our Own (AO3). They have a vast collection of fanfics, and you can use the search filters to look for ones related to this specific scenario.
Annie is often depicted as a more reserved and calculating character. She keeps her emotions in check and is very strategic in her actions. Mikasa, on the contrary, is more passionate and driven by her love and loyalty. She is protective and doesn't hesitate to jump into action when needed.
In fanfiction, Mikasa and Jean often have their characters explored in different ways. Sometimes, their relationship can be developed into a deep friendship. Jean might be shown as having a newfound respect for Mikasa's strength and determination. This can add new layers to their interactions within the fan - created storylines.