I wonder if it's possible to take the head out of American football, and use helmets as just extra protection in accidents - like helmets in almost every other endeavor (climbing, construction, bicycling, etc.). Make it a game played from the neck down.

You can count on people saying it's impossible, 'this isn't football', players saying 'I can't play this way; I'll retire', etc. That happens with every change, including the ones that all the players accept and play with now, and which are part of the most popular sport in the U.S. Not unlike responses to changes in technology.

Already, things like helmet-to-helmet hits, spearing helmet-first, etc. are against the rules (I don't know the exact rules). Players can control what body parts make contact with what on the opposing player - obviously: in other things in life, including other sports, you don't instinctively lead with your skull.

I would guess that blocking between the offensive and defensive lines would be the biggest change. They begin inches apart and slam into each other at the start of each play, and generally whoever gets leverage lower will win. I think it would just look different, more arms and less body-to-body.

Also, are there sensors for helmets that would pickup impacts? I think sensing the accoustics / vibrations might be sufficient.

They have mouthguards now with kinematic sensors to measure collisions. It's called the NFL Instrumented Mouthguard Program.