throwing a nano in my car soon 1) I think an upgraded voltage regulator on the VIN pin would be my #1 "want".
2. The next one would be better 5V and 3.3V regulators for more stable voltages on my digital pins
3. and as reference on my analog pins (right now I can't rely on the accuracy of my ADC readings).
The 3rd thing I might want is higher quality oscillators
the ability to draw 20mA on a few pins without experiencing voltage depression.