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.
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.
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.