It would be handy to be able to toggle ruler lines that you can set a start and end point which to help doing designs so they are symetrical and the rulers (or guide lines). There could also be circle shapes that can be incremented bigger or smaller.
Additionally these shape guides could also be used to fill or place voxels where it sits on the grid as well, so you can easily create circles, triangles or lines X making this tool useful either to create shapes or a visual guide of the shape only as you create something. Hope this makes sense?