You could go with the "standard" herringbone gear design, but add some gearing to the interior shaft that are inlaid and overhung some, so there's no practical way for it to be produced in a reductive or casting method. Don't think there would be a practical application for this, as you couldn't get a usable piece to fit inside. Not sure of the WOW factor for the average person, as they wouldn't understand the complexities of the design.

Another idea would be an orbital gear, fully encased, made in one piece, then have another that is still functional but cutaway with a clear cover on it so people could see the intricate structure that was a single build.

And the Da Vinci, even with all it's sins, isn't that bad for me, been a work horse so far.