Some kind of Rulers are already missed in Black Ink.
On thought of mine are to allow SVG vector graphics import for that.
This would offer a lot of flexibilitie in shapes of Rulers.
The paint program "Rebelle" did this already, almost any Brush follow
such a Path. Here is an example image, even if you import a pattern
consisting of several Paths into Rebelle, a Brush follows each individual Path.