Offset the stroke on a curve

[RocketPoweredTennisBall 3]

what i'm thinking is another slider on the stroke modal window that controls how much the stroke is offset. on closed curves it would read outwards on one side and inwards on the other, or something like that, and i'm not sure for open curves, but it would be leftwards along the normal vector when moved to the left and rightwards when moved to the right.