A future version of the YouTube app for Android gets the option to automatically download videos so they can be viewed offline. This is evident from an analysis of the app by Android Police.
New YouTube app with “Playlist Auto Sync”
10.35 version of the YouTube app already includes an option to delete all offline videos and playlists from your device. In addition, in the near future it will likely have a function called ‘Auto Playlist Sync’. The teardown of the APK file indicates that videos that are added to this playlist, are made automatically offline available. This happens on all paired devices that are connected to WiFi. The user has nothing to do for this.
Playlist Auto Sync function is still hidden and can only be switched on by developers. Playlists can even be made available offline, but must be changed manually.