The ONLY way to make sure the intelligence agencies to work for the American people and not for a faction of Americans is to make it mandatory that a group of five private citizens (per agency) are put into observational, oversight roles. Each of the five should be from separate parts of the nation, have the absolute highest security clearance (equal to or more than anyone else in government) and have the capacity to monitor everything that the each agency does. This should be a revolving position that changes every four years, but is staggered so that there is always overlap.
These people should have no connection to government, and no connection to anyone in government. They must have been employed regularly for at least 10 years prior to coming to the agency, and they should not have any security clearance issues.
It should be made clear that these individuals are extensions of the average American citizen, and as such any plotting or attacks on them will be considered an attack on the average American citizen.
On its face, that sounds like a pretty good idea.
Their information would have to be kept secret so they could not be bribed/blackmailed.
The OIG is the closest we have to anything like you described. Do you Remember what 44 did to Gerald Walpin.