A permit from the U.S. Patent and Trademark Office suggests that the pencil could potentially work on other surfaces as well.
According to the letter, the Apple Pencil could in the future work with optical sensors across a wide range of surfaces, potentially including the iPhone, MacBook Pro, Apple Watch and in other devices. Who know maybe on paper or wood too?

The patent describes a stylus that could be capable of detecting movement, tilt, rotation and position in 3D space. Which sounds pretty epic.
That basically means it wouldn’t rely on interacting with the sensors within the iPad, but would have its own sensors that would capture motion and translate that into digital input without needing to touch a screen.
The same patent also describes how the Apple Pencil could have a ball-shaped stylus that rolls as you move it. This makes it sound almost like some mice, but unlike traditional mice that use internal wheels to detect the rotation of the stylus, the patent says it would use optical sensors.
This would apparently make the surface less vulnerable from scratches and contamination.
While this would be a natural fit for devices like the “Vision Pro” or future headsets, where input tools need to work beyond a flat touchscreen, it also suggests that Apple isn’t done exploring what else it can add to its vision for the spatial computing era.
For now, Apple Pencils only work with iPads and they do the job very well.