A continuing frustration with the mouse is that it requires lifting one's hands off the keyboard to use. Not only is there that extra movement, but the place your hand must be to use the mouse is usally not so ergonomic: on a laptop with a touchpad you bend your elbow a lot, on a desktop your arm is a little too far out to the side (especially if mousing righty when there's a number pad). But what if you could use the keybord as a mouse?
Make the tops of all of the keys be tiny touchpads. Figure out how to tell typing apart from mousing. Then you just slide your fingers around across the keys. For large movements you can cross many keys, for precise ones you stay on the same key. This makes more sense with flat keys like a laptop has than the scalloped ones many desktops use. Clicking still needs a way, but that could be two new keyboard keys below the space bar that you press with your thumbs.
I think the two tricky parts are distinguishing typing from mousing and making it cheap enough.