In the Attack on Titan manga, Armin is still alive. His character plays an important role and continues to be part of the story's progression. There are many close calls, but he manages to come out unscathed.
In 'Attack on Titan' fanfiction, the relationship between Armin and an OC (Original Character) can be really interesting. Usually, the OC might be designed to complement Armin's personality. For example, if the OC is brave and impulsive, it can contrast with Armin's more calculated and initially timid nature. This contrast can create a lot of engaging storylines, like when they face Titans together. The OC could also be a source of support for Armin during his times of self - doubt, which is a common theme in the 'Attack on Titan' universe.
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.
Another theme is growth. They both grow as characters throughout the series, and in fanfictions, this growth is sometimes shown in relation to their relationship. Maybe Armin becomes more confident and Mikasa learns to open up more because of their bond.
The OC can also bring new relationships into Armin's life. This can create new conflicts or alliances. For instance, if the OC has a family member who is against the Scouts, it can cause tension between Armin and the OC, but also lead to interesting story developments as they try to resolve their differences.
The contrast between their characters. Annie is strong, stoic and a bit of an enigma, while Armin is more cerebral and empathetic. Their interactions can be really fascinating to explore in fanfiction.
One popular story might be about their unlikely friendship evolving during their time in the military training. It could explore how Annie's tough exterior starts to crack in the presence of Armin's gentle and intelligent nature.
There's also 'Armin and Mikasa: A New Chapter' which focuses on their emotional connection. It shows how their friendship might develop into something more. In this fanfiction, they are often seen having deep conversations and relying on one another in difficult situations.
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.