Longer battery life, if I had to pick just one. However, that probably can't be changed at the moment. And I was getting better battery life on my second G4 than my first, which is encouraging.
In terms of improvements that could likely be made to the existing hardware, allowing faster charging, even as the phone's temperature rises, would be very nice. When it's charging in the car, and the battery is STILL going down (due to the throttled-back charge current), that's not good.
You need to get a different car charger, or use a decent usb cable.