I believe the next generation of full sized iPod will look a lot like the iPhone. I think Apple will want to take advantage of all the R&D that went into the iPhone. I would expect to see a multi-touch screen, maybe an extra couple of hard buttons and of course to run OSX. I don't think it will have WiFi (and thereforee features like web browsing, widgets, googlemaps, email, etc) though. I think Apple will want to keep the lines between products distinct.
I also think that the Nano will remain basically the same except have more memory. As the Nano gets more flash, it will be the iPod of choice for people who are only interested in just audio and for those who are concerned with size.