We have HP Indigo's that can print white ink. For example, printing white ink on a golden paper, it looks great, and if you have a picture, just slab some white ink below it and it looks good.

However in InDesign, if you set the colors accordingly (Paper color to gold, and add a white spot color), and you enable overprint preview, the white ink just disappears. This means, we're desining with paper set to white and white ink set to Cyan to see what we're doing. Tbh we'd like to see what we'll eventually get.

We need the ability to set the Solidty of the spot colors, like Photoshop is able to since many many years (see screenshot, comparing the Spot color dialogue between InDesign and Photoshop [german version]). Also, we need the ability to export either a version with or without the paper color (with paper color simulated to show the customer how it'll look like, without for printing).

There are many printing systems out there that can do white ink and other ink with high solidity: HP Indigo, Xerox Iridesse, Ricoh, ... also, screen printing, and even certain metallic colors in offset printing!