Within one year, Apple have always included in the iPod touch the major hardware updates that help developers. Such as the retina display, the dual core chip, and the 4 inch display.
If the next iPod touch isn't updated in another two years, it will need a 64 bit processor (A7), the bigger iPhone 6 display, and Touch ID. So that is the 6th gen iPod touch.
When? This fall, one year after Touch ID and 64 bit. It's not coming next spring because that would be detrimental to Apple's profits, which revolve completely around hardware sales. And it would be idiotic for capable supply chain manager Tim Cook to let a child-oriented product launch skip the holiday season.