Some years ago I heard an interesting proposal on a podcast. I have no idea where I heard it so can't give credit where credit is due, but I wonder if anyone is thinking seriously about this strategy.
Require all police officers to purchase their own liability insurance. Each officer would be given a stipend based on the cost of said insurance for an applicant with a "clean" record. Anything above that amount comes out of the officer's pocket. If an officer is perceived as a bad risk by the insurance companies, their price would presumably increase. A really bad cop would not be able to afford the premiums and officers would have an incentive to minimize those behaviors which increased their liability costs.
No idea if this is realistic or even possible but it seemed like an interesting idea when I first heard it and seems even more interesting now.