In my opinion, what the world of tablets need is a good tool for quick and reliable one-hand writing, which would be a) easy to learn, but b) quicker than handwriting. It would most likely not be a physical product, but a replacement for a standard keyboard.

There were a number of solutions -- like chorded keyboards -- but none has really approached the popularity of traditional QWERTY; I believe that with software instead of a physical keyboard many more solutions might happen in the future, some even becoming our new favorite text-entry solutions.