Don't limit it to pass interference. Sometimes there are just obvious bad calls, like the two hands to the face calls, but there is nothing a team can do about it. You get 2 a game, and lose a time out if you are wrong. Then there should just be a sensor in the tips of the football for goal line situations (or first down situations) that are aligned with the clock and you should be able to see what time a knee went down, and quickly compare it to where the sensor was when the knee went down. Final change is to put the ball on a yard marker on every first down and get rid of the stupid chains. My kids' flag football league has a better way of determining first downs than the NFL.