There are a ton of different streaming services and other sources of music out there. In order to make it possible for us to support as many as possible of them we’ve designed Mopidy to be extension based.
In practice this means anyone can go to http://docs.mopidy.com/en/latest/extensiondev/ and start adding their own favorite source of music. Typically what is actually needed is some way of getting the URLs to audio or some way of getting raw PCM audio through the providers APIs. If these basic conditions can be met there is a fair chance you can make a new plugin.
Our Extension wishlist label has a bunch of requested extensions which are just waiting for someone to come along and create them. Ideally the author(s) should be someone with access to the service in question, someone who will use it so they notice breakages and also importantly someone how intends to maintain it. None of these are absolute requirements, but more suggestions.