Two pictures that went through my head while reading this (and a third older one revisited):
An adjustable mount at the top of the extruder that holds a simple micro switch, where you turn a screw to move it closer to the filament. The mere presence of filament closes the switch. That’s help when plastic ran out, but not so much with jams where you want to know whether the plastic is moving.
(older idea I and others have had for years): laser print onto a label a visual encoder image in the shape of a 608 bearing. Stick that to the side in the idler, with a small optical sensor to see whether the idler is moving and in which direction. That’s detect jams, and might almost replace the need for #1
with cheap enough and small enough circuit-board based cameras like the raspberry Pi board they announced, have an extruder designed to hold a mini camera pointing exactly at the hobbed bolt digging into the plastic. Video stream is saved for diagnostic purposes too. Good for watching via human live (just a cool and useful view), plus you might design video analysis algorithms that detect problems (looking for powdered filament caking the teeth), or replace #1 and #2 above with simple edge detection… (If you don’t see the notches on the filament coming down the bottom, it may have ground away.
All of these seem like a lot of work for only some benefit (we do get our printers printing without it, after all), but once the work is done and published as a new model (and firmware using that data is made public), then everyone can benefit.