The police and prosecutors need to work closely together to function. It's unreasonable to ask the prosecutors to then also prosecute police powers. It's biased and unjust—even when the actors are all doing their best to act in good faith.
We currently allow PDs to investigate themselves, which is a mockery of justice.
It ends at subpoena and arresting power. The courts would mediate disputes as they already do.
People testify because they feel greater loyalty to society as a whole than to the defendants.
Neither of those things are going to work against cops.