Ecash is a nice invention for trusted environments of which on a grand scale of things there are currently only a few places where it makes sense.

I would love to see someone build a Monero ecash mint.

So e.g. for a conference you could buy your ticket with Monero and get some ecash tokens for fast and easy and private spending at the conference, where you likely trust the organisers. And then after the conference the mint gets closed automatically and any leftovers get stored privately on the Monero chain again.