After reading numerous articles about football helmet design and the comments that "there will never be a concussion-proof football helmet," I am concerned about my grandson who plays high school football. I am also an aerospace materials engineer who does not like the term "never." If we subscribed to the adage that we would "never go to the moon" we wouldn't have much of the technology that we have today.
All of the articles that I have read about football helmet design focus around making the inside of the football helmet energy absorbing and always keeping the outer "shell" hard and rigid. There have been experiments with soft layers over the outer shell but they tend to just relocate damage to the neck.
Let's look at it a different way. How about making the SHELL energy absorbing. Keep the ridged shell there so that it can be used for attachment of face masks but make cut-outs in the shell so that it can flex somewhat and absorb energy. The shell material itself could be made a little thicker and more pliable, so that it can flex without cracking. Remember, the hardest thing that a helmet has to resist is another helmet, so if ALL helmets were designed with energy absorbing shells both would absorb the energy of the impact.
Attached is my rendition of such a helmet. The holes are similar to that of a Wiffle ball but they could be made into any shape or distribution. It's also possible to make slots versus holes. The designs of the energy absorbing slots and holes are only limited to the skill of the engineer since virtually any configuration is easy to manufacture. While you are at it, make the outer surfaces of the helmets non-stick (slippery) so that when two helmets hit each other they slide to one side or another rather than twisting the necks of the impacting players.