And what would be the incentives of such a prosecutor? They're appointed by, say, a mayor, or a governor - if they do a "good" job of prosecuting brutality, ar

And what would be the incentives of such a prosecutor? They're appointed by, say, a mayor, or a governor - if they do a "good" job of prosecuting brutality, are they going to get a further appointment to a judgeship, or get enough support from the police union to pursue an elected office? How are they going to get good staff when they can't promise them any advancement inside that office, and doing too good of a job precludes them from being employed in a proper prosecutor's office? Effectively all you've done is established an organization specifically dedicated to sweeping up messes.