My current "wild guess" for the next iPods are:
1) iPod shuffle--will stay the same form factor but will go to 4 GB and 8 GB versions.
2) iPod nano--will stay the same form factor but storage capacity will now be 16 and 32 GB flash memory. I'm hoping for a slight increase in thickness to accommodate a better video camera and a rechargeable battery with longer life per charge (maybe up to 35 hours per charge).
3) iPod classic--possibly phased out, but could switch to a larger 16:10 aspect ratio screen with higher resolution with smaller click wheel and either 160 GB hard disk or 128 GB flash memory.
4) iPod touch--will be based on 4th generation iPhone (including iPhone OS 4.0), with storage capacities of 32, 64 and 128 GB flash memory.