My opinion we won't be seeing much larger harddrives. They cost a lot, and i think Apple is going away from the 'Your Whole Library in Your Pocket' and are going to make a push for iTunes to be more of a library manager. I think we'll see a lot more customization/ease of what we toss onto our iPods.
I think we'll see a 24gb in late 08, and it will have very few new features that can not be done on the current touch