One possible plot could be that Armin is kidnapped by a group who oppose the Scouts. They might want to use him as a bargaining chip against the Survey Corps. Maybe they hope to exchange him for some valuable information or resources that the Scouts possess.
Another possibility is that the kidnappers are a group of foreign spies. They come from a land outside the walls that has its own interests in the situation inside. They believe that Armin holds valuable information about the Titans and the technology within the walls. So they kidnap him to take that knowledge back to their own people.
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.
In many fanfics, Annie and Armin's relationship can be complex. It might start from a place of mutual suspicion as they were on different sides initially. But as the story progresses, they could find common ground. For example, they both have a deep understanding of the cruel world they live in. This shared understanding can lead to a more empathetic connection between them.
Another theme is his relationships. His friendship with Eren and Mikasa is frequently explored in different ways. Some fanfics might focus on how his relationship with them changes as they all grow older and face different challenges.
A Jean x Armin fanfic often explores their unique relationship. It might focus on their growth as characters in different scenarios. For example, in some fanfics, they could be on a mission together and develop a deeper understanding of each other's strengths and weaknesses. This type of fanfic can also delve into their emotional bond, perhaps showing how they support each other through difficult times in the fictional world they inhabit.