What does pen direction refer to, what's it calculated from? Is it hardware identical to stroke direction?
And pen rotation is Wacom art-pen specific, if I'm correct? It's very brand/model-specific and I believe patented at the hardware level so only Wacom users can use it currently.
For caligraphy and such I wonder if it's possible to feed angle from a touch dial because many other branded tablets now have those, in that sense rotation of the brush can be emulated by having your thumb on the touch dial plus perhaps with some shortcut for activation. It'd be useful as I know quite a few people without rotation.
I believe you can also use tilt to emulate rotation a bit, tilt that has a progression in the axis e.g. 30deg to 50deg to 70deg can show say a clockwise direction as it's a generalised heading in the same sense as stroke direction is. That could be "memorised" and previewed as the current state of the brush rotation. It wouldn't be perfect rotation necessarily but it would allow caligraphic dynamics, a sort of tilt-induced twist. It could be called "Pen twist" or something and perhaps useful for even those with hardware pen-rotation.
Pen tilt can also provide dynamics like direction-by-lean, or is that what pen direction is? That would be essentially be the half-product of pen-twist.
Another form of dynamic...
Pen jump may be something to consider for flick-pen splatters, etc. Where jump can work with velocity and stroke direction to create jumping splatters and sprays. Jump can be defined as how long the pen has taken in the air before it touches back on the canvas. I saw that Microsoft did this in the old Surface demo and it seemed effective but no program has ever used it. Of course the brush stroke ends when you lift off, but the stroke is still preserved as the last vector in memory I imagine? So then when that is set, the pen jump is continuous until the brush fails to land within a set timeout in milliseconds.