Apple's really holding tight on to those A5 devices, even though they don't play nice with iOS 8. Unusual for them. The Touch used to get updated every time a new iPhone came out. Usually with the same internals, but with a crappier camera. The current one has only been out two years but it's now three full hardware generations behind.
If they update it early 2015, I can see it having the same guts as the iPhone 5S. Probably the same form factor with the 4'' display. If it comes out late 2015, it might have the same internals as the iPhone 6, though likely also in the smaller form factor. My money would be on the 5S based model though.
I do have a feeling that Apple may be wanting to phase out the stand-alone iPod line. Both the Nano and the Touch haven't been updated for two years. Shuffle hasn't seen any attention in four, and the Classic was just discontinued. Apple TV hasn't gotten an update in awhile either, though Apple has never really taken it seriously. I'm not sure how well the Touch still sells, but since the smartphone market is becoming so saturated, I can see demand for these devices weakening.