1. I've wait for a study of case from you.
You still owe me a case study where your idea would be better than the existing one.
2.
In all other 3D programs, manipulation is represented by a coordinate system
I can't see major differences between "other" and the current one:
3. If you want a customized pivot point, you will first need to define the 3 coordinates of this point and then set the rotations to it.
If you're thinking of just setting it with the mouse or your fingers, forget it. This is not a professional approach for someone who wants at least some precision.
4. If you don't like to use plugins (ok) BUT don't suggest to unnecessarily complicate the main program.
PS. I'm a master of useless (YAUP).
On Multiplier versions prior to 2.0 I've set 8 extra pivot points.
Then, even a dumb like me realized their uselessness and removed them.