How about a high capacity Bluetooth pocket server compatible with iOS?
Streaming from cloud storage is useless when you don't have network coverage, and when you have data limits, which is most of the time for most people.
For people who want to have more than 64GB or possibly 128GB of music and videos on the go, there is still no replacement for the Classic.
But imagine a device that's just a 250GB micro hard drive, a Bluetooth chip, and a battery, able to serve all compatible iTunes files to iOS devices. It would work with the iPhone, the iPod Touch, and the iPad, and could be a "local cloud" storage for apps, music, videos, photos, everything. You could keep it in your pocket or your bag, and it wouldn't add any weight to your other devices. The battery could easily outlast the battery of the Classic. It doesn't have to be paper thin, so it could have a larger capacity battery.
It's easy to imagine such a device replacing the Classic!