I entered a new ER, under "features" because I'm not sure if a helix is a curve feature or a sketch element. It should be a sketch element but SW doesn't treat it that way. Anyhow, I was working on an answer for Robin (https://forum.solidworks.com/thread/62325?tstart=0) and was frustrated. I think a two-line definition for a helix wiould be the bomb.
This is the ER:
Define a helix by two lines, where the user selects one line to be the helix axis and selects the other line to be a tangent to the helix. The two lines (theoretically infinite) must not share a point and must not occupy a plane that is perpendicular to the other line. SolidWorks assigns a start point to the helix at the tangent point. The user must select a direction for the helix to continue from the tangent point and must select either the number of turns or the degrees of turn.
This feature would be quite useful to create a transition from a straight line to a helix that reflects design intent. It is very difficult to impossible to create this with the current tools. (As ever, if there's a good way to do it with the current tools, I'm happy to learn.)