Add curved path primitives that can be used by other tools, lath for example, and plugins. The path object that would consist of vertices joined together by some sort of spline curves, and could form an open or closed path.
This path object could of course be manipulated in normal 2d/3d space as usual.
Depending on the spline type, there may be extra control points that can be manipulated, as well as the vertices themselves.
Ideally, there would also be an option to convert the spline path to a normal path primitive using a number of options:
a) convert to N straight segments.
b) convert to a minimum number of straight segments according to some resolution (screen, or unit measurements perhaps) .