I don't see why football should be singled out when boxing is still legal. The object of boxing is to hurt your opponent - knock them out or at least beat him/her into submission. The object of football is to score points, not intentionally injure opposing players.
Just as it is now illegal to grab face masks, other rule changes could limit hits that produce injuries: players should wear lighter helmets/less less protective equipment. Heavy helmets and padding only encourage harder hitting. Also, limit the number of substitutions during a game. That way players would be forced to pace themselves - and there would be less incentive to bulk up to 300+ pounds.