Casting to Sonos speakers would be pretty great, it would mean I could go from my living room (with no sonos speakers - just kodi through my sound system) to my kitchen (with a sonos speaker) and back and never miss a beat.
Also, maybe music from other sites (youtube, vimeo etc) could be cast (currently not available through the Sonos app). And again, I could go from watching a concert to listening to it in my kitchen to watching it again.