incredible long battery life. (what about the new capacitor batteries that reload in seconds)
I would dig a rollable display, that "hardens" when its rolled out.
And you shoud be able to roll out A5 for reading books and further to A4 for magazines and newspapers.
would definatley need a touchscreen, but that in rollable will probably be in the far future.
textrecognition for handwriting.
Dictionary support (touch a word and it shows a translation)
color display
awesome bookmark handling