Imagine a pack without any main compartment, just many smaller compartments attached to a framesheet. You'll say its weight-inefficient due to extra fabric, but you don't use any stuffsacks, dittysacks, bearhang sacks, first aid/repair bag, stove bag. All of those various bags get attached to a framesheet.
The sum of all of those little bags (most of us carry a few at least, even if its a ziploc) plus the harness they go on, is the weight of the bag. Nothing is "inside" your pack. It's modular: not bringing your stove? Don't attach the stove compartment. You save the weight of the Cubic inches that stove takes up too.
I'm just spitballing. This would probably be 100% MYOG, custom size pouch for each personal item. But it could incorporate a few purchased drybags, one for quilt, one for clothes, etc.
You'd probably want to add an empty bag (on-trail water grabs, resupply, worn clothes).
I think water in something like the Nalgene canteens fastened directly to the framesheet would carry very well, close to your spine and vertical.
What got me thinking this direction (very related but separate concept)
I recently hacked up an old camelbak into a minimal harness. The concept is to attach drybags for as a quick modular day-pack. The harness is 6oz so a bit heavy for this concept, but it carries well enough with a 20L bag/10lb test load (jacket, pants, lunch and 3L water, upper limit for sure).
Inherently waterproof. Rolltop closure bag clips into the top of the harness (supports all weight right at base of spine onto shoulders). Then I used the sternum strap to wrap around the bottom of the drybag (compression/stops swaying). I'd compare it to the Gossamer Gear Riksak in function (though 2x the weight), but likely way more comfortable in the 3-10lb range. Again, its a concept (hacked up camelbak) that I'd like to make from scratch closer to 3oz.