"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
AirPods and AirPower: Everything We Know
Apple to Update iPod Touch with 4-Inch Display, A5 Chip, Buffed Aluminum Rear Shell?
According to asian source, new iPod touch (5th generation) will have 4 inch LCD which is the same size with coming iPhone 5.
CPU or other spec is considered to be same with iPhone 4S, and has different back-case design, which will be not mirror polished but buffed alminum one. And it seems to be prepared black and white color model.
Rough mockup of fifth-generation iPod touch with buffed aluminum rear shell and larger display
The report's source also indicates that there is an additional hole in the lower portion of the rear shell, separate from the camera found on the current model. It is, however, currently unknown what the function of the hole will be.