My idea is to implement a hover state to the shuffle button, allowing listeners to shuffle through their library by choosing a distinct mood. Often I find that while shuffling music my mood plays a large part in what I want to hear, hearing a pop song come on while I'm in a relaxed or forlorn mood is distracting, and conversely, hearing a sad or lumbering song while I want to play dance music is also annoying.
I realize that one could make playlists with similar music types, but this implementation is faster, using machine learning to serve up music quickly based on mood, instead of requiring the listener to do all the hard work. Also, I feel that shuffle options would give Spotify advantage over other streaming services, implementing music listening customization is what can set Spotify apart from its competition. Attached is a mockup of said idea.