I personally think I would like to see this enhancement, but think it would take some getting
used to. Being used to drawing lines that way coming from Autocad to Pro-E to Solidworks
I am kinda used to doing it this way. The rectangle is nice, but still not sure about the line.
You can constrain a centerline to the midpoint of a line and then contrain the centerline
to a plane or axis, but like you said, it is a lot of clicks.
Interesting idea. It would be helpful.
About the closest you get to this right now is by turning on the "Infinite Length" option when the line command is active. Of course, you still have to add the midpoint relation.
I don't know that I would like it as I do a variety of things with lines. However, you may want to look into dynamic mirror as a way to sketch a bunch of stuff and have it mirror as you go. This would be the same functionality that you are desiring only with a bit more setup. But it also might help in what you do.
That would only work for lines perpendicular to the mirror line. Anything else will give you two lines.