Improve the range and accuracy of song info.
Maybe we're a bit biased because we're sticklers for information since it's what we deal in, but we hate seeing incorrect info on Spotify, especially the wrong year of release. The service needs an army of fact checkers who make sure things are in order. Or, at the very least, let users fix data on their own. It works wonders for Wikipedia.
It wouldn't hurt to add information like "genre" and "producer," either. Those are both qualifiers by which we'd love to search and organize our libraries and playlists.