This is simple but a game changer. I’m praying someone can slip this into the next service release.
On the _CurveThroughPolyline command, I’m asking for an option to _Join input curves. Currently, if you select two deg1 curves with 7 control points (deg1/7) and ask for a _ControlPoint deg3 curve, you get two deg3/7 curves. History is enabled for this command, so any changes to the parent deg1/7 curves are reflected in the deg3/7 children. Here is an example of the current functionality using deg1/3 curves that are quad mirrored:
The requested _Join option would take the two deg1/7 curves and look for coincidental endpoints. If one coincident is found it creates a single open deg3/13 curve. If two are found, it creates a single closed deg3/12 curve. If none are found, it works as normal.
This is the simplest way to achieve another useful history enabled curve symmetry tool. Here is the same deg1/3 control curve pictured above if the proposed _Join option existed. The edits to a single parent deg1/3 could then propagate through a number of children created by mirrors, arrays, etc. to update a single deg3/11 grandchild curve: