I don't know much about this yet, but I've heard a few mentions of Spotify opening up an API to allow third-party programs access. If Mixxx were the first out of the gate to incorporate this, it would be a major win over VDJ and Traktor. Basically, for the price of a subscription, your library would become virtually unlimited. Of course, the track analysis would have to be on a just-in-time basis, but the results could still be stored in the Mixxx database for future use as long as you were to load up the same copy of the same song. I'm thinking that a Mixxx DJ with an internet connection would almost never have to say "Sorry, I don't have that with me".
There may be some legal restricitions involved, of course. I expect that disallowing recording when this or similar APIs are is in use would be required.