Easy: no helmets, no pads. Play American football in rugby gear. If defensive players can feel the pain, they won't be able to turn themselves into human missiles. Lots more bruises, lots more contusions, infinitely fewer concussions, fewer ruined careers and lives.
I love football, but had I any children, I would probably forbid them from playing it. You are perhaps not as likely to die as you are in, say, auto racing, but it is becoming more and more likely that you will be sacrificing your brain and overall wellbeing to the sport.