I know this is probably asking too much, but any death or serious injury under police control should immediately go to a grand jury. With a grand jury, they have power to subpoena any and all evidence, including the stuff that the police like to hold back. And they can also compel people to testify under oath, which encourages the cops involved to turn on wrongdoers where they would normally be content with sitting back and not saying anything. Also, since grand juries do not decide on guilt, there is no 5th amendment right to abstain from self incrimination. They must tell the truth or risk being held accountable with severe consequences if caught in a lie. In other words, take those weak internal affairs investigations that are staffed by brothers in blue, and put it into the hands of a jury and give them some powerful tools to get to the actual bottom of the matter.
The best part of the idea is that cops would be forced to justify the reasonableness of their supposed reasonable use of force, and they would be held to the standard of "reasonable" that the public decides, not themselves. And it is not a trial, it is merely a screening for a trial to see if it passes the bar for criminal action.