Here’s one for you then.
Architectural drawing frequently needs to establish a centreline or centrepoint between two other lines/points. There used to be an extension called Midpoint that helped with this.
So how would it work?
For points, I imagine invoking the command and then picking any two points in space to define the controlling parameters. Then the extension would place a marker (3D cross?) at the centrepoint with axes normal to the working environment. Ideally, this would be a species of construction line. In fact it could be three such lines meeting at the midpoint. That way, it is easy to delete it after use by clearing all construction lines.
For lines, something very similar. In this case you would select two parallel and coplanar lines and the extension would place a single construction line halfway between them. There would have to be a warning system if the two chosen lines were not parallel or were not coplanar.
In case it’s useful, what I tend to do for lines using native tools is to place one along one of the “founding” lines, then copy it to the other founding line, then hit /2. It’s pretty simple but a bit of a faff.
STOP PRESS*
On reflection, maybe the lines don’t need to be coplanar, just parallel?