As for me, I would start with a Treo 750. This is the best form factor I have found yet.

I would start by upgrading the processor to the fastest available, and upping the memory 4X over what is currently available.

I would then switch the screen for a 320 x 320 (400 x 400) touchscreen model and add WiFi capability.

Next, I would bump out the bottom corners to accomodate 2 more keys on the bottom row to fully accomodate a telephone keypad (# and *)

I would move the vibrate/ring switch to the left side of the device below the three buttons, and move the IR port to the top.

Then on the right side of the device I would add 4-5 user programmable buttons.

I would cook up some better hinges for the SD card slot.

Finally, I'd add a star trek style communicator cover that would fold down over the face of the device - with openings for the phone keys, the 5 way and keys beside it, and clear cover over the touchscreen.

This to me would be the perfect device.

What would you do?