I would dig a folding touchscreen design similar to this.

Beside the advantage of having a device that can stay pocketable like the current device; there is that ability to pull a DS-like two screen effort.

Given the announcement of the Hapitkos technology from Nokia and the placement of the tablets as N-series devices, I would see something like this added to that two screen concept in order to really push that idea of interacting (over simply inputting).

Addition: I would have only a few main buttons: power, and zoom +/-. These buttons would be universal in function and that's it. There would be two additional buttons, but aided by the tech, they would shift their mode according to context (similar to Moto ROKR E8).

I would also hope for the E-series engineers to get in there and fine tune the mess out of that hardware (man that E71 is a heck of a kit).

Finally, OLED for both screens to increase the ability for the device to chew less power and give a better screen.

Come to think of it, this can be done now...