The 'ultimate pack' descriptions fit the new Lowe Alpine Zepton type packs almost perfectly, you might take a close look at how they built them. The only main difference is they use a wire, steel u frame, which is very light and incredibly strong, I had to stand on it to alter the bend even slightly. But everything else in terms of the basic structure is exactly as described. They use a stiff but shape holding plastic plate on the hip belt, and also in the pack itself, with a roll of padding on the top part of the hip belt to fit your body's bend, and only 3d mesh, about 1/8 inch, on the rest of the hipbelt, and that's totally comfortable. Now I see why they did that pack the way they did, they were trying for something very close to what is being described here as desirable features of an 'ultimate pack'.
Lowe Alpine by the way has also used that same double aluminum stay, external, that the above pack shows, in two channels, big lumbar pad, etc. That design really works, but I think their new one is even cleaner, not sure on the durability so far of course, only time will tell.
This thread, coupled with my zepton's living examples which will be easy to copy and use as templates, is helpful to decide on my own first pack design, which will be frameless and smaller (don't need another 35 pound 50 liter capacity pack, the zepton already does that fine, at 2lbs without top, 2.5 pounds with).
I'd add to the ultimate pack list, if it wasn't already mentioned, the main drawback of the zepton, side pockets that are too small, in terms of how wide they open. So I'd make at least one side fit one or two full either nalgene or platypus type liter bottles, and using solid fabrics, not mesh, like ula very wisely just switched to. No snags from brush etc.
And I'd make the compression cords go under the pocket, and add a cinching cord or strap over the pocket to hold smaller stuff in snugly. Still can't decide on a top pocket, for easy construction, using a roll top is a no brainer, but I'm not sold on it being the best solution, so I might add straps for a top and decide after, or make it removable.
Oh, and more attachment points, grossgrain loops etc, than the zepton, especially on the straps / belts and back panel, it doesn't have enough, I find that limiting. And a rear net / mesh pocket (can't get used to calling the rear pocket the front pocket, heh.., it's in the rear, not the front). Maybe removable, that might be slick, or not.