Krita has a framework to allow painting assistant, it is the virtual version of physical tools such as rulers, compass... It allows to restrict the drawing to follow a given shape. It differs from the line/ellipse tools in that those tools gives perfect and regular results, while the assistant allows the artist to have some control on the drawing. The following blog post contains a video that show it in work:
The tool to manipulate painting assistant is limited, it allows creation and edition. What would be nice is if the user could save on the harddrive the current configuration. And then load it. This would be implemented by adding two buttons to the tool configuration, one for save, one for load. Using a xml format.