If I could design my ideal disposable, I would make it soft and flexible when dry. Since to do that the SAP would have to be less than mostly used today, there would be extra fluff, making the diaper thicker.
The shape would fit my body which is petite. I would be happy if it were as long relative to my body as is a baby disposable on an infant. There would be soaker material in the front side wings. There would also be a short section of soaker on the back flaps so that on me the side soakers meet without either a gap or over-lap.
Because my ideal disposable fits me correctly, it would be practical that it only have one wide tab per side, like a scaled-up baby disposable. The outer layer would be poly-plastic like that used circa 1985, meaning the tapes need to be sticky and not re-positionable. The inner feel-dry layer would be soft, without the institutional sensation of some modern disposables. To me feel-dry is not all that important.
The decorations would be printed all over the cover, like modern baby disposables, using generic cartoon characters.
For my normal bedtime I would love pinned diapers made of gauze. These would be trapazoidal and just the correct length to reach slightly above my hips and the back of my butt. The soaker would be made of non-woven synthetic sponge such as Kodafil, but this would not extend all the way toward my waist in front or back. The front side flaps would be narrower than my front. The back side flaps would be wide enough the pins fit in the hollow of my hip bones so if I am on my tummy I do not feel the heads of the pins. To keep this adult pre-fold comfy I would use 2 pins per side, one set at the waist and one to control the leg holes.