Solar panels, e.g. Apples's patent for a screen integrated one, contactless inductive charging, pixelqi style sunlight readable display, e-ink style display which uses no power to keep a static display, display privacy with a display that can change it's viewing angle right down, make the back optionally touch sensitive as well, if you remember Doc. Brown from back to the future had some augmented reality binoculars, you could speculate about possible camera improvements which would aid that kind of thing such as infra red sensitivity to help apps locate people in scenes, haptic feedback more than just 'vibrate', and again very speculatively dynamic changing materials - e.g. If the handheld device could change texture, kinetic charging, pico projectors, pressure sensitivity both on the screen and on the device generally, someone made a nice hack to wear a magnetic ring so they could gesture above their phone and sense it using the compass - theremins can do similar using some kind of em field distortion, could a device integrate anything like that without a huge aerial?

I'm not sure if you're saying there's nothing else we could put in a device now in hardware terms, or if there's nothing else interesting we could put in a handheld device ever, or just nothing we could put in that a consumer would want, but I'm not convinced for any interpretation.

its better when you decouple hardware and software.

Citation needed.