The Worldshaper's Sextant is cool and all, but creating spheres/domes is just as annoying as creating a circle, if not more so. The sextant is meant to be an easy way to create circular things in builds without having to tab out to a guide, but the fact that it doesn't do spheres/domes means that half of the time you'll be doing that regardless.

How this would work:
Shift-left clicking (or some other keybind) with the worldshaper's sextant, will swap it between "circle mode", and "sphere mode". In circle mode, it will act as it normally does, creating a circle. In sphere mode, it will create a sphere (wow who would've guessed that), again, centered at the block chosen, with a radius defined at where you look. Switching between circle mode and sphere mode while an imaginary circle/sphere is already created, will also swap the current selection from a circle to a sphere, or vice versa.

As a cute touch, maybe the sextant in sphere mode could look a little different? idk what it would look like though.

I have gotten express permission from williewillus to post this feature request here.