Modularity: Specified power connections and optical data busses allowing the components to be stacked, and mixed and matched. All of the components would also be a matched size, say 8.5 x 11".
The screen would be what we're now calling a tablet computer. When connected with the system, it would operate as a screen, but it would have functionality when it was on it's own. It was conceived as a palm pilot on steroids, able to do basic tasks, but with a lot less computational power than the main CPU. It would have a virtual keyboard for when text was entered and would be, of course, a touch screen.
There would be a main CPU module. It would be pretty much like the base of a regular laptop computer, and would have bays for hard drives, optical drives, networking etc.
Batteries would have two formats. One would be similar to current laptop batteries, another would be very thin but the same size as the standard components, able to be stacked up.
I initially conceived the keyboard as stackable with the other modules, but now, it may make more sense to have it connect with bluetooth. These two, however, are not mutually exclusive.
A similar design concept that I've kicked around is a modular cell phone. The core functionality of the cell phone would be a unit as small as possible. It would not even have an integral mic and speaker, but in that form would only be usable with a wireless headset. If you're out on the town, and want as little as possible to carry, this would be it. It possibly, in that form would be able to receive calls, but only able to make calls using voice recognition.
That phone core could be plugged into various devices, be it a matching PDA to turn it into a smart phone, or even a large handset optimized for use as a telephone. Likewise this phone core could be plugged into the modular computer system I describe above to give connectivity to the computer system (3G etc) and could use a bluetooth headset, speaker phones, or bluetooth connectivity to the big handset to use it as a phone.
If I had access to an R&D department like Apples, this is what I'd have them design. Unlike Apple, I'd make it a publicly available standard (though I may retain the rights to update and change it), much like the IBM PC standard.