I had an idea for making more precise distinctions using a fixed two-finger position and a tap with a third finger.
The two fixed fingers would serve as a reference plane, while the third finger taps in different positions relative to them:
Tapping directly next to the two fixed fingers
Tapping farther away from them
Tapping higher up
Tapping lower down
The reference plane would be defined by the two stationary fingers and could shift depending on the angle at which the hand is positioned on the trackpad. This way, the fingers wouldn't always need to be aligned perfectly horizontally to establish a reference plane.
This could, for example, replace the four arrow keys.
A good setup would be to keep the middle and ring fingers fixed while using the index finger to tap. It's easier to tap farther down with the index finger than farther up. Therefore, the dividing line between the upper and lower zones shouldn't be at the same height as the middle finger, but slightly lower.
The reverse setup would also be conceivable, with the index and middle fingers held fixed and the ring finger used for tapping. This would be better suited to less frequently used gestures, since lifting and tapping with the ring finger independently is somewhat more difficult.
This could be also work with one finger fix or 3 finger fix.
