1G and 2G iPods had 1200 mAh batteries, but the new ones have only 630 mAh. Increasing the power capacity to even 1000 mAh would help, I don't think it would increase size or weight too much. A bigger "RAM" (cache, buffer) would help battery life too, so it wouldn't access the HD so often.
Someone asked if the batteries were Li-ion or lithium polymer. The answer is: 1G and 2G iPod batteries were lithium polymer, but the new ones are lithium ion.
Replaceable batteries would be a plus. If they could make battery life 10 hours, and give you the chance of getting a second battry, that would make 20 hours. Or if you're battery has ended it's life, you wouldn't have to void the warranty by doing the changing itself, or taking it to iPodResQ.
New colors, like in iPod mini would be okay, though I wouldn't mind if the iPod would look more like the original one, the "pro" one. Click-Wheel is absolutely welcome. Though, if the iPod has colors, and a Click-Wheel, it's no more an iPod, it's just a bigger mini. Or maybe it's meant to be like that. Only Steve & Co. knows.
Color screen, not bad, video playback and picture viewing would be nice, if it wouldn't exhaust the battery too quick...