One interesting 'Joker Black Butler fanfiction' could be about Joker's backstory before he joined the circus in the Black Butler universe. Maybe it delves into his past traumas and how he came to be the person we see in the series.
Fanfiction.net is also a good place. There are many Black Butler fanfictions there. Look for the specific tag or keyword related to Joker. However, the quality may vary, so you might need to read a few to find the really good ones.
Joker might be a key figure in a mystery or crime plot. His knowledge of the seedy underbelly of the circus world could translate into knowing about illegal activities in the modern underworld. His ability to deceive with his performances could also be used to his advantage when dealing with criminals or those trying to uncover his secrets.
In modern time fanfiction, Joker's character could be adapted as a circus performer in a more contemporary circus setting. He might use modern technology in his acts, like using LED lights in his juggling or something similar.
Definitely not. The Black Butler series has a set of established themes such as the supernatural elements, the Victorian - era setting, and the complex character relationships. The idea of a 'leashed slave story' is completely foreign to the official canon and is likely a disturbing creation outside of the proper series. It would go against the overall tone and moral framework of the official work.
There is indeed a 'Black Butler' visual novel. It features the same dark and alluring art style as the anime and manga. The visual novel might include different storylines, some of which could be exclusive to this format, providing a new perspective on the 'Black Butler' world. It often has a strong focus on character development and the complex relationships within the story, which is great for fans who want to explore more about the characters.