Modbook has been doing this for years. They put a Wacom digitizer on top of a display installed over the bottom half of a Macbook Pro to make a tablet: http://www.modbook.com
I’d much rather Apple creates an iPad Pro. For instance, a 13.3" screen @ 3072 by 2048 pixels could fit two regular iPad apps (in portrait mode) alongside each other. Then, apps could be adapted to take advantage of the double screen real estate (looking at you, Photoshop Touch.)
As for pressure sensitivity, Apple doesn’t need to build that into iPads, Wacom already sells pens for iPad that have 2048 pressure levels and palm rejection. [1]
Imagine partnering with Wacom and making a portable OS X Cintiq...