I think if you go watch 6 man football at the high school level in the states that play it (Texas is one; a couple of other states play it as well rather than 8 man played elsewhere), you'd see the collisions I'm speaking of. You have players running full speed and no downfield blockers to either slow them up or cause them to run around the blockers, making for potentially pretty rough collisions. Put 2 or more guys at the size and speed of NFL players and you could have a disaster.

Think about how NFL defensive coordinators will scheme a wider field. More zones and more safeties. The hits may be legal, but as we know, injuries happen more on legal hits than they do on illegal ones.

I'm just suggesting they think about this.

Then I suppose the other way to reduce high speed collisions would be a shorter field, but it'd probably have to be a lot shorter to have much effect.

I have some other ideas for reducing the energy of collisions:

Longer grass/turf
No shoes allowed
Sumo suits
Increase the number of players per team to 33
Play in a swimming pool

Others?

Yeah, thicker, softer padding on everything, including the outsides of helmets. It doesn't technically reduce the energy of the collision, but dissipates it over a longer time or impact. The main problem is that the better you protect the head, the greater the danger to the neck.

Or you outlaw pads & helmets entirely, and the players may (or may not) reduce the energy of their collisions.

Softer playing surface, as you suggest, might help too, as in mandatory muddy field. Wrestling mats would be very effective but very expensive; used as is for football, they'd probably need replacing every game.