New extruder concept

That's not even close. That's just a flex drive shaft to typical extruder.

Maybe I'll have to draw something up, but what I am talking about is the filament being driven by the twisting of the flex shaft. Imagine an internally threaded screw biting into the filament. By turning this screw, the filament is driven forward or backward. This happens along the entire length of the shaft which results in the filament getting full engagement all the way through the drive system.

So, flex shaft is basically like a nut, filament is the bolt. Turn the nut, and the bolt moves forward/backward.

The only moving parts are the stepper drive on the hub of the filament reel with a tap-off to the flex cable. At the hot-end end of things, nothing is moving and no added weight or gearing. I imagine backlash would be minimal since the filament is fully engaged the entire length of the flex shaft. It would also facilitate autofeed systems for quick filament changes.

lol. Can you picture what I am talking about now?

Not feasible. It would chew up the filament, and flex shafts are lubricated. Running filament down the middle would both contaminate the filament and remove the lubricate from where it belongs, in the flex shaft.

Not to mention the tremendous backlash and friction you'd be introducing.

EDIT: and what IronMan said.

Drawback to bowden is nonlinearity/backlash, and inability to print flexibles.

Well, I don't think I should abandon the idea outright, honestly. But good to identify what/why it wouldn't work and determine ways around that.

We know from direct-drive that the main advantage is more pressure as close to the hotend as possible which gives us the most control over feedrate. The drawback is the extra weight from the steppers/gearing and pulling on the filament can sometimes affect accuracy. The extra weight also limits how fast we can print and change direction.

So, this is where I am coming from. Remoting the extra weight, but allow the filament to be driven right at the hotend.

Instead of the flexshaft engaging the filament the entire length, what about only at the very end of it?

Instead of hard threaded drive, we might be able to do a spring like coil sharp enough to bite into the filament. This would maybe help with varying filament diameter. Having the driving area smaller like the last 10mm might help reduce twisting.

The main issue I see is that you would just twist the filament until it breaks. Here's what I mean: If you just turn a nut on a screw without holding the screw in place (stationary), the screw will just rotate together with the nut. In order for the screw to move, you need to counter act the torque you apply on the nut. So there are two issues: 1st, how do you intend to keep the filament from just rotating with the threaded shaft. 2nd, even if you can somehow prevent it from rotating, it will most likely just twist off.