1. First the battery compartment should be redesigned to properly fit some easy-to-find high capacity cell phone battery as main choice, not "almost fits", "a little too long", "goes in if you force it" etc. Proprietary batteries make devices useless once the batteries wear out.
2. Bluetooth is more important than wifi, since with bluetooth the Nano can easily tether to a phone, use as a music player with bluetooth headphones or car audio system, etc.
3. Really good quality audio electronics for music player use would go well in portable audio enthusiast circles.
4. More radically, get rid of the color screen and go with reflective monochrome like the old HP 100LX. Much better battery life, and works outdoors.
Usefulness of the Nano form factor isn't so clear anyway. What I really want is a Kindle-like e-reader running the Nano software, all GPL, with a small keyboard and running on commodity AA cells like the above mentioned HP 100LX.
The 100 LX was the greatest pocket computer ever made (given the technology of the day) and you should play with one for inspiration.