What would it be. This would be as a carry on and for doing something like the AT, as well as for just bumming around the world. Lets say you could take one change of clothes, a micro puff and a rain/wind breaker w/pants. With this, a shelter, pad, bag/quilt, pot and stove. It would have to fit the carry on requirements. Something like 22x14x9 inches.
You would want it to be able to carry a load of water comfortably, if need be. At least a gallon, maybe two, for those sub Saharan locals, or the desert SW.
I have this ten year old Columbia panel loading travel pack, that has tuck away shoulder straps and hip belt, that came with a zip off day pack. It was horrible to carry with the day pack on it, but with that zipped off, surprisingly, it's not too bad and it fits the carry on size requirements, has a frame sheet and aluminum stays, but it's sort of heavy, probably in the 4-5 pound range.
I'm just curious what others would come up with, if faced with a single pack challenge.