What about allowing you to stream music you've already bought from the iTunes store directly from the iTunes store server?
Of course, I could store more music on my iPod than I've bought from the iTunes store, so, neato but useless.
I have 25 years worth of CD purchases on my Mac, plus a few dozen songs from iTMS
But if we discuss streaming from the iTMS, then Apple _could_ create a subscription service. I suppose music subscriptions is one of those things that Steve Jobs would never, ever even consider in a million years, up to the day when he changes his mind and the service is there