Spotify and Sonos are definitely the two big apps which would be brilliant to have. Not holding my breath though for them anytime soon, Spotify took forever just to optimise the iOS app for the 6/6 Plus.
I agree, it was a painfully long time before Spotify upgraded their iOS app, so since it hasn't been mentioned for the Watch, I'm not going to be holding my breath. But I wonder if the watch will be able to have generic control over whatever app you have on your phone playing audio, be it Spotify, Audible, or Stitcher for example. These can all be controlled with the Control Panel on your phone, so can the Watch also do the same without having an app specific widget?