It's hard to say. 'Levi x Eren' fanfics are quite common in certain fandoms, but the 'Levi is a criminal' aspect makes it a more specific and perhaps niche sub - genre. If there are a lot of fans who like to see dark or morally ambiguous takes on the characters, it could potentially be popular within that subset of the fandom. However, it also might not be mainstream popular as it goes against the more typical heroic portrayals of the characters.
In the 'Attack on Titan' series, there is no clearly defined canon romantic interest for Levi. His character is mainly focused on his role as a soldier and his fight against the Titans. However, many fans like to ship him with various characters in fan - made love stories.
One possible theme could be unrequited love. If Simahin is in love with someone who doesn't return the feelings, it would be a very sad situation and unrequited love would be a fitting theme.
The cinematography in the movie also helps present the sad love. The use of lighting, for example, can create a mood of melancholy. When Gatsby and Daisy are in some scenes together, the lighting might be dimmer, symbolizing the gloominess of their relationship. The settings, like Gatsby's mansion which is a symbol of his unfulfilled dreams including his love for Daisy, all contribute to the overall theme of sad love.
I recommend the book "My Talent is Resurrection." There were many sad elements in the story. The protagonist, He Luqian, had the ability to be immortal, but this did not mean that he would not be tortured by pain and pain. He constantly fought against fate for love and justice. It was very touching. I hope you like this fairy's recommendation. Muah ~😗
Maybe Levi is a criminal in the sense that he's part of an underground rebellion against an unjust regime. Eren, who is initially on the side of the establishment, meets Levi and gradually comes to understand his motives. Their relationship could develop as Eren has to decide whether to side with Levi or stay loyal to the regime, all while a romantic connection forms between them.