Proposal for non-circular bubble.
Not sure if this is doable or you are interest in implement this but would like to cover the entire surface area of the touch screen and not leaving any gap. This would increase the touch area for easier selection.
For round watch, a circle in the middle, which leave a donut on the edge. We can subdivide this donut into smaller quadrants depending on how many “bubbles”.
For a square watch just make everything square/rectangular and do a best fit algorithm.