Well, writing a good 'Black Butler Shinigami Fanfiction' requires you to be immersed in the world. Get to know the existing Shinigami characters like Grell Sutcliff and William T. Spears. Then, think about how you can expand on their stories or create new Shinigami characters. When it comes to the plot, it can involve the Shinigami's relationship with the other supernatural beings in the Black Butler world, like demons. You can also add some historical or cultural elements from the Victorian era, as the series is set in that time. This will make your fanfiction more rich and engaging. Also, use descriptive language to paint a vivid picture of the Shinigami's actions and the settings they are in.