If they do it won't change much, probably just A6 and a slightly better camera and native iOS 7.
I could say the same thing with the iPhone: The 4S just has A5 vs A4 on iPhone 4 and it has a better camera. You're pretty much right, but it not only could have A6 but also 1GB of ram, an iPhone 5 camera but better.
And I recently thought about my previous pattern which every new gen. will support one iOS version more, I think I was wrong. Every iPod Touch supports three different systems:
1st gen supports iOS 1, 2, 3
2nd gen supports iOS 2, 3, 4
3rd gen supports iOS 3, 4, 5
4th gen supports iOS 4, 5, 6
5th gen... iOS 6, 7 and 8.
That's why 6th gen must be released in 2014, so before WWDC 2015 will announce iOS 9 then 5th gen will be dead.
SO: Don't expect 6th gen come out this september.