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.
The theme of discovery is also common. They might discover new aspects of each other that they hadn't noticed before. For instance, Armin might discover Mikasa's hidden vulnerability, or Mikasa might find out about Armin's secret dreams and desires, which in turn deepens their connection.
Some Tumblr blogs dedicated to Attack on Titan fanfiction also post armin x mikasa stories. You might need to do some searching around different blogs to find the best ones.
A really popular one is 'The Promise'. It delves into a world where Armin is injured and Mikasa takes care of him. This situation makes them both confront their feelings that have been hidden for so long.
Some Tumblr blogs are dedicated to sharing and discussing fanfictions. You might be able to find mikasa x armin fanfiction there. Look for blogs that are focused on the Attack on Titan fandom in general.
Well, 'Under the Stars' is also a notable one. It takes place in a more peaceful setting where Mikasa and Armin can finally focus on their relationship. The author does a wonderful job of depicting their love in a very tender and believable way.
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.
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.
People like it because it offers a different perspective on their relationship. In the original series, their friendship is strong, and fanfiction takes it to another level, exploring romantic possibilities.
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.