Themes like family and found family are quite common. Batman and Nightwing have a mentor - protege relationship which can be seen as a form of family. Harley Quinn, who is often an outsider, sometimes gets drawn into this dynamic. Power dynamics are also a theme. Batman has a lot of power and influence, and how Nightwing and Harley Quinn react to that, whether they challenge it or submit to it, is often explored in these fanfics.
One great 'batmanxnightwingxharly quin fanfic' is 'The Unlikely Alliance'. It shows an interesting dynamic between the three characters. Batman's brooding nature, Nightwing's acrobatic charm, and Harley Quinn's chaotic energy come together in a story full of action and unexpected alliances.
Blade Fangs was a Japanese manga created by Itagaki Keisuke. There were four works in total, namely Grappler Blade Fangs, Fanma Blade Fangs, Blade Fangs Path, and Blade Fangs Path 2. The theme of this manga was hand-to-hand combat, and it told the story of the young Blade Fang fighting with strong enemies in various underground arena competitions. The characters in the Blade Fangs series had distinct personalities, compact storylines, and unique styles. This series of comics was well-known in China and was welcomed and loved. The animated version of Blade Fangs also had two seasons and could be watched online. As for the viewing order of Blade Fangs, they could be viewed in the order of Blade Fangs 1, Blade Fangs 2, Fanma Blade Fangs, Blade Fangs Path, and Blade Fangs Path 2.
I'm not sure specifically what the'sama sama letter day story' is about as it's not a widely known common story. It could be a very personal or a story from a specific local or cultural context.