Another theme is growth. Fanfictions often show how both characters grow as individuals and as a pair. Tamaki might become more confident with Mirio's influence, and Mirio could also learn important lessons about himself through his relationship with Tamaki. It's all about their mutual development.
One place to find such fanfiction could be Archive of Our Own (AO3). Just search for 'amajiki tamaki x togata mirio' on the site and you'll likely get a good number of results. There are various genres and themes within these fanfictions, from friendship - centric ones to those exploring deeper emotional bonds between them.
My Hero manga features Mirio prominently. His character adds depth and excitement to the narrative. His abilities and challenges he faces are integral parts of the story's progression and appeal to readers.
Some fanfics focus on the theme of support. In a 'male x mirio' relationship, they support each other through difficult times, whether it's dealing with personal problems or challenges in their hero - related tasks. It shows a really positive and heartwarming side of the relationship. And of course, there are also fanfics that play with the theme of the power dynamics between the two characters, which can lead to some unique and exciting plotlines.
A common theme is their bond. Since they are friends in the original story, fanfics often explore how deep that bond can be. It could be a story about loyalty, where they are always there for each other no matter what.
Comic Vine provides a lot of details about Mirio. You'll discover his skills, his role in the overall plot, and how fans react to him. It's a great source to understand this character better.