Taking the ultralight, long-distance hiking paradigm of reducing effort and achieving multi-functionality further, we can arrive at some pretty wacky and unconventional ideas.
Everyone here knows that 15 lbs on your back is a heck of a lot better than 30 lbs on your back. Okay, but how about 15 lbs on a walker or rolling trailer in front or behind you versus 15 lbs on your back. No contest.
What we need is a super-light aluminum-frame back with wheels on the bottom. Whenever the trail becomes smooth, take that puppy off and pull/push it on its wheels. The frame unfolds to become your tarp/tent frame.
Need electrical power for your cell phone or flashlight? The dynamo mechanism puts the brakes on the 'backpack-cart' on downhill sections, saving your knees and storing tons of energy for later use - for instance, powering the heating elements in your special 8 oz blanket that is good down to 40 degrees. If you want, you can even sit on the pack and roll downhill with it, increasing the energy produced by 3-4 times. Why not?
So much potential energy is wasted on endless downhill sections. Surely there must be a way of harnessing some of it!
I could see these kinds of ideas actually catching on in Europe, where so many elderly people do lots of fitness walking on smooth, civilized trails.