Armin could be the storyteller. Sometimes characters take on this role to provide a unique viewpoint or to drive the narrative in a particular direction. But we need to know more about the story to confirm it for sure.
Armin is still alive in the manga. The story developments don't result in his demise. His presence remains significant and continues to shape the narrative.
A Mikasa x Armin fanfic often explores their unique relationship. It could show their deep emotional bond that might have developed over their shared experiences in the story. For example, their mutual understanding of the harsh world they live in and how they support each other through difficult times.
One possible scenario could be that in the fanfiction, Armin and Annie's pregnancy is a result of their deepening relationship post - the end of the main story's conflicts. Maybe they find peace together and start a family. Their different personalities, Armin's intelligence and Annie's stoicism, could create interesting dynamics during the pregnancy, like how they handle the news, their preparations for the baby.
Armin is also usually shown as a character with great determination. He doesn't give up easily, no matter how tough the situation is. He might be facing seemingly insurmountable odds, but he'll keep fighting and trying to find a way forward.