Yes, there are quite a few. You can try searching on fanfic websites like Archive of Our Own. Just type in 'Female Titan x Armin' in the search bar and you'll get a list of relevant fanfics. Some might be really well - written with interesting plot twists and character developments.
Power dynamics is a frequent theme. The Female Titan has great physical power, while Armin has intellectual power. Fanfics often play with how these two types of power interact in their relationship. Another theme could be redemption. Maybe the Female Titan is seeking redemption through her connection with Armin. It could also be about understanding the 'other'. Armin tries to understand the Female Titan, who is different from him in many ways, and vice versa.
One common theme is the exploration of the blurry line between friend and foe. Since the Female Titan is initially an enemy, fanfictions often delve into whether she can become an ally through her interactions with Armin. For example, Armin might try to convince her to side with the humans.
One popular theme is the growth of their relationship. Starting from distrust or indifference and evolving into a deep friendship or something more. Another theme could be them facing challenges together, like a particularly difficult Titan attack where their combined skills are put to the test. Also, the theme of sacrifice might be present, where one is willing to sacrifice for the other in a life - or - death situation.
One popular trope is the 'enemies to lovers' dynamic. Given their positions as former enemies in the main story, it's common for fanfictions to explore how they transition from hostility to love. Another trope is the 'hurt/comfort' where, for example, Annie might be injured or in distress, and Armin is there to comfort her, which deepens their bond.
One popular theme is their friendship evolution. As they grow up in the dangerous world of Attack on Titan, their bond changes and deepens. Another theme could be sacrifice. How far would Eren or Armin go to protect each other? And there's also the theme of trust. Given the complex and often deadly situations they face, trust between them is crucial and can be a major focus in fanfiction.
Well, in many fanfics, a popular plot is when Eren and Armin have a disagreement about how to deal with the Titan threat. Eren might want to be more aggressive, while Armin advocates for a more cautious approach. This leads to a rift between them for a while. But as the story progresses, they realize they need each other and come back together stronger. Another plot could be them exploring a new area outside the walls and encountering new types of Titans or mysteries that they have to solve together.
One possible storyline could be that Armin discovers a way to communicate with the Female Titan on a more empathetic level. He tries to understand her motives beyond just being an enemy. Maybe she has some hidden knowledge about the Titans that Armin needs to save humanity.
One interesting fanfiction could be where Mikasa and Armin go on a secret mission together outside the walls. They face many challenges and in the process, their relationship deepens as they rely on each other's strengths. Mikasa's combat skills and Armin's strategic mind work in harmony.
One common theme is loyalty. Armin and Erwin are both extremely loyal to their comrades and to each other. In fanfics, this loyalty often becomes a central aspect of their relationship. Another theme is leadership. They are both leaders in their own ways, and fanfics might explore how they lead together or support each other's leadership.