iCloud Time Machine for Mac. Much like the iCloud backup that's available for iOS, but for the Mac.
Apple could still be smart and media files like mp3, Mac App Store apps, the whole set of OS files, etc would be treated like they already are for iCloud syncing. Apple doesn't need a million copies, one for each person who downloaded a song, they need one copy linked to a million accounts. It's only each person's individual and unique files that would have to be stored--just as they already do with the iTunes Music Match for audio.
All as automatic as Time Machine, but instead of the backup being on a USB drive, it's all up in the cloud.