Another theme is mutual understanding. Despite their different personalities, they seem to understand each other on a level that others don't. Fanfics often play on this, showing how they can communicate without words at times. For example, in some stories, Levi can tell when Hanji is about to do something reckless just by a look in her eyes. And Hanji can sense when Levi is more stressed than usual. This mutual understanding forms the basis of many emotional and plot - driving moments in the fanfics.
Another one is 'Hanji's Perspective on Levi'. This fanfic is unique as it is told from Hanji's point of view. It gives readers a different look into how Hanji sees Levi, their quirks, and the special connection between them.
There could be various fanfictions. Maybe some that focus on alternative endings, like ones where certain characters survive who didn't in the original. For example, a fanfiction where Erwin survives and leads the Scouts in a different way.
One way is to check on popular fanfiction websites. Look for ones with high ratings and a lot of reviews. For example, Archive of Our Own (AO3) has a large collection of AOT fanfictions. You can search for the most popular ones there.