I'm not sure about the extremity armouring. Maybe a "light" armour does not need that much total coverage of the body at all. I was pointing more at the back covering part of the armor.
A stiff plate of ceramics and/or kevlar as a front shield and something a bit more flexible (splitted plates?) for the back is still light in meanings of shrinking the agility of the soldier only by its weight, not by blocking any joint.
Additionally some kind of stiff protectors for the shin bones and forearms would give more protection than a standard lighte vest without being too heavy?
By the way where in the tech tree shall these advanced armour types appear? Will alien tech improve them? That should be part of the design then I guess.
edit: What about a stronger helmet? I guess the one you used right now is the same of the standard light armour? Maybe a closed helmet with some kind of visor would be cool.
But please do not make it look like a bike helmet unless you provide us a cool kawasaki or quad model as well...