One great Loid x Yor fanfic is 'A Spy's Heart'. It delves into the deeper emotions of Loid as he starts to truly care for Yor beyond their fake marriage setup. It shows his internal struggle between his mission and his growing feelings.
One characteristic is the exploration of their hidden emotions. Since they are in a fake relationship at first, the fanfiction often delves into how they really feel deep down. Another aspect is the family dynamic. Given that they are part of a fake family in the original story, fanfics may expand on how they interact as a family, like how they take care of Anya. Also, the setting of the story, whether it's in the middle of a spy mission or just their daily life, plays a big role in making the fanfiction popular.