Mastering Engineers still face a tightly closed black box when it comes to predicting Spotify's playback. There are some resources and plugins that predict the outcome, but nothing is more accurate than the real thing. And the real thing can change. It has.
I have at least 40 (and growing) Mastering engineers, who would love to see some type of "test" account access to Spotify to test out new masters.
Either for free or a low, monthly fee, Mastering Engineers could upload unreleased music to a separate, but accurate Spotify test engine that will not only allow them to test their own masters directly on Spotify's engine for quality, dynamics, and loudness, but allow the engineer to hear already released music alongside the test master by way of private playlists. The test music would not be "released", but rather, only playable in the Mastering Engineer's Spotify test account. After the test is complete, the music can be immediately deleted, or passed onto the artist/label, to simply accept, add meta-data and artwork, and then release.
At the very least, would it be possible in the near future to use test accounts to play unreleased music on Spotify's engine? After all we're here as Mastering Engineers looking to improve the quality of music on Spotify.