Perhaps one day I'll simply design my own board and have it fabbed but for now I'm better off buying off the shelf complete boards that have already been designed because I'm buying something that has been tested and I know what to expect.
Since I'm throwing a nano in my car soon, I want it to be able to cope with alternator voltage, which can swing from 9V to 16V with possible transients outside of that range.
I think an upgraded voltage regulator on the VIN pin would be my #1 "want". The next one would be better 5V and 3.3V regulators for more stable voltages on my digital pins 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 and lastly, the ability to draw 20mA on a few pins without experiencing voltage depression.
Also, there is something strange going on in my car whereby if I turn off the arduino before I turn off the car, the car goes into "limp mode" which means it cuts power by about 80% or so and runs like shit. The reason it does this is because the throttle body is drive-by-wire and there are 3 throttle sensors (one for the pedal, one for the commanded plate angle and one for the actual plate angle) and they all have to agree within a narrow tolerance. Somehow the Arduino is sending feedback through the sensor wire and throwing off the voltage on one of the sensors when I turn the Arduino off. I don't know if this is due to the quality of the board or it's just a design constraint but all in all, I feel there's more to be desired in terms of electrical stability and I was hoping that there would be a 3rd party board that had addressed this by using more expensive components that don't fluctuate or burn out as easily.