I'm surprised no one has mentioned this in this massive thread (unless they have and I missed it), but a tablet is a perfect place to implement a shapewriting input system, where you drag a single finger over keys on a keyboard to "draw" words rather than stroke each key individually. This would allow easy one-handed entry, something that will be crucial for a larger-format device. The software is already available on mobile devices and Windows-based tablets via 3rd-party software, but Apple would be wise to integrate this functionality at a system level in their upcoming tablet (if indeed it exists).