A music-making marketplace. What do I mean? Somewhere online that lets people sell simple musical components (like melodies, beats, lyrics, chord progressions /backing tracks) and musical services (like singing parts, mixing, playing, sequencing, arranging). Whole songs could be assembled in this entrepreneurial fashion.
How would the transactions work? Let people set there own terms (maybe support auctions), but one idea I had was everyone involved in the final track gets a cut of the songwriter royalties. If it's a hit, everyone could get rich! And sell the finished products on the front page, obviously (unless people opt to take it offsite).
If nothing else, it'd be something a bit different. Songs are often made like this anyway... Like the latest Eminem song was a beat from Alex Da Kid, probably using samples of something else, which Eminem put verses to, then a hook from an unknown singer was added, which was then rerecorded by Rihanna, etc