Wouldn't that be a novel idea?
We keep speaking of animations. However, animation in of themself are canned. Therefore, any encounter that triggers an animation will therefore have a predetermined outcome.
A true physics model would produce random outcomes and take the speed and mass of players interacting into effect. That would make player to player interaction much more realistic; not only in tackling, but in blocking as well.
This would take a lot of horsepower to do, but I bet its possible on nextgen systems.