There are definitely some interesting ones. One aspect that makes some of these fanfics great is how they handle the challenges that a trans Armin Arlert might face in a world as harsh as that of Attack on Titan. It could be discrimination from other characters or her own struggle to come to terms with her identity while also dealing with the life - threatening situations in the story. Some fanfic authors do an excellent job of integrating these elements into the narrative, creating a rich and engaging story.
Archive of Our Own (AO3) is a great place to start. It has a large collection of fanfics on various topics, including ones about Armin Arlert as a trans woman.
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.
First, understand Armin's character well. Know his strengths, weaknesses, dreams, and fears. Then, create a believable scenario for his death. It could be related to the overall plot of the original story or something new. For example, if the original story has a lot of battles, his death could be a result of a strategic sacrifice in a crucial fight.
Eren might be shocked at first. He has always seen Annie and Armin as comrades, and this sudden pregnancy news would be quite unexpected for him. He might be a bit hesitant but eventually come around to support them.