I would like 3D Curve features. These curve features would be capable of being used as guide curves/paths for sweeps and lofts as well as be able to be projected into sketches.

Intersection Curve - creates a curve between the intersection, or projected intersection, of 2 (sur)faces.
Projected Curve - creates a curve based on a selected sketch, curve, or edge as projected normal to the target (sur)face.
Helical Curve - create a helix or spiral curve.