I've thought a lot about luggage design ever since, twenty years ago, when I decided to commit to traveling carry-on only.
In the beginning, I used a ca. 2000 RS Convertible backpack which was right at the carry on limits for most airlines, and I pretty much filled it. Its empty weight was about 2½ lbs.
As I traveled more, I identified things I didn't need to take, and as I acquired dry-overnight clothing, which allowed me to wash as I go, thus carrying fewer clothes, I needing less space. After 4 or 5 trips, I switched to an Outdoor Products (now Campmor) Essential Carryon, which was about 10% smaller, and ¾ lb lighter at 1 lb, 13 oz.
I really loved the OPEC bag (which is not longer offered), but as I continued to reduce the amount of stuff I took, it, also, became too big. With no cinch straps to tighten the load, the bag sagged badly, and I wanted a smaller bag with cinch straps. I had also noticed I was having to hold the top of the backpack straps to keep them from sliding off my shoulders, so I wanted a sternum strap.
About that time, when I returned from a trip, I put everything from my OPEC bag in a box of known length and width and measured the depth. The contents of the box was 1400 cu inches. That was about the volume of my Appenzell backpack. I tried it and everything fit, but I didn't like the “backpack” (from the top) loading, so I kept looking.
By this time I had begun to formulate a set of requirements for the ideal carry-on bag.
1. Basic rectangular solid with no tapering and minimally rounded corners,
2. One big compartment, any pocket is full sized, avoid pocket proliferation,
3. Opens like a book for ease of packing,
4. Hide-away backpack straps
5. Cinch straps
6. Sternum strap
The first two requirements mostly maximize the volume within the dimension limit imposed by airlines. Since the volume of my contents is now about 50% of that limit, 1 and 2 are no longer essential for me, but for someone who can't resist the urge to over-pack, they would apply. Pockets that are less than full size add to the overall dimension without adding much usable volume. Avoiding pocket proliferation helps keep the weight down.
I finally found an eTech 2.0 Convertible Weekender Jr. bag on eBags. I've used it ever since for trips both to Europe and domestically, and I love it. It weighs ¾ lb more than the OPEC bag, but the cinch and sternum straps more than make up for the added weight. Unfortunately, eBags no longer carries that bag. It's replacement (mentioned above) weighs about 3½ lbs (PIG!).
Most of the bags suggested above weigh more than what I would consider acceptable for carrying. However, in checking them out, I did come across a bag that I really like. That is the Hynes Eagle 38L travel backpack. It weighs only 1.76 lb If I were in the market now, I would definitely buy one. It is not sold on eBags, but it is sold on Amazon. According to its published dimensions, it's actually only 33L, but it is enough under the airline carryon limits in every direction that it can probably be overpacked to 38L without being too large.
Hynes also has a 44L version which is just slightly smaller than regulation carry-on size.