There is definite "method to the madness" for making UI changes, but
afaik there hasn't been a definitive location for outlining the
general guidelines on UI changes. Perhaps the best place would be in
the Inkscape Developer's Manual, so I've started a section here:
Please review and add. I don't think we need exhausting details, just
explain what the general motivations/principles are that Inkscape UI
developers should strive to follow.