I think there is a miscomprehension here:
Chromecast should be not just a Chromecast Audio thing. @maxmp correct me if I'm wrong, but Google's Cast Protocol is the same across all devices and it would include Google Home, Chromecast, Android TV, Soundbars with Google Assistant and more stuff like this.
The only "challenge" is to deliver an UI to Chromecast (I don't really know if it is required to have Google's approval). With that done, it will be a cross-device thing, cause it's pretty much universal.
