I think the game has legitimately gotten much more dangerous, and I think the cause is that everyone has gotten much bigger, heavier and faster. The force the guys get hit at now has increased dramatically. Remember the 85 Bears and what a novelty it was that the Fridge was 300+ lbs.? Now 300 lbs. is common and they are all athletic also.
I heard Mike Ditka on the radio once - he said if you take away the modern helmets, people would quit using their heads as weapons.
Another idea is to have a weight limit by position.
Not sure there really is an answer but you can't fine-tune the rules enough to make the danger of the game more like it used to. It's hard enough to play defense anymore, and it's changed from being a running back dominated league...if you can't hit the QB anymore or mess with the receiver, why bother running the ball?
Just my opinions and ramblings...