Backpacks at 20 pounds makes so much sense! The only problem these days is that the wonderful new backpacks may weigh that much empty.
However, my current favorite is an Eagle Creek convertible. It has padded shoulder straps &a very substantial waist belt. These both tuck very cleverly away in zipped compartments. There is even a little padded flap under the bottom which, when you use the bag as a backpack, protects your back from the wheels. It has wheels (big, skateboard wheels) &a pop-up handle which permits rolling. There's a zip-on day pack too.
If you don't cram it full, it will slide through the carry-on template at security stations now used at many airports, if the zip-on day pack is removed. I usually carry on the day pack &check the larger bag because I hate the jockeying for bin space on aircraft these days. I travel internationally 3 or 4 times a year, domestically usually 2 or 3 times &have used the bag for 3 years &it still looks pretty good. The only delayed luggage problem I've ever had was a 2-hr wait in Edinburgh last month.
My previous backpack was the old Rick Steeves model. No wheels, no waist belt ¬ nearly as comfortable to use on my back. The melamice chewed big holes in it on an Australia trip one year, at Crocodylus, so I had to replace it.
Gail in Eugene