Bill Gates says pen-based tablets will beat the iPad, at least with students.
Something like a decade ago, one of my favorite rant topics was ‘where oh where is the electronic brush?’ I was referring not to brush gadgets such as are common in drawing programs, but to a physical brush that could be used in conjunction with an electronic display. At the time I imagined a brush composed of optical fiber, along with some internal electronics to detect the hotspot of a CRT as it passed beneath the bristles, and more electronics in the computer itself to correlate the signal generated by the brush with a position on the screen.
Well, CRTs are hard to find these days, and LCD screens don't have hotspots. On the other hand, touchscreens have become quite common, and at least those used by Apple are good enough to use for drawing.
The trick would be to find a bristle material, or the combination of a bristle material and the internal design of the brush, that would sufficiently mimic the capacitance of a finger to be detected as such. You can use as fine a brush as the touchscreen will reliably detect.
The difference between such a brush and a pen-input system might appear negligible, to the casual observer, but would be far more pronounced to the person actually using the device, due to the difference between the gradual contact of a brush and the sudden contact of a pen. Moreover, a brush would provide pressure information directly to the device, via the touchscreen, without need for a Bluetooth connection, and clues to nuanced movement via the rolling or rotation of the triangular area contacted by the tip, so you'd be able to use it to move sliders or rotate dial gadgets using very small motions of your fingers.
Also consider that the written language used throughout east asia is traditionally drawn with a brush, and is still more legible done by brush than with with a pen.
Besides which, there's an elegance to brushes that no pen can match, and the main reason for using a hard pen, multipart forms, simply isn't a consideration on a touchscreen.