Analog gauges are beautiful. I have never seen the Air Fuel Ratio gauge option that Markos shows (Lambda gauge) and for a carb'ed car it is probably the most useful one.
A body roll gauge would be awesome and have an airplane feel to it.
Electrical, I suspect current is more relevant than voltage to anticipate issues. Coolant temp, well there should be one at the bottom of the radiator and another at the thermostat I guess.
Yes, yes, I'm feeling a gauge deficit. Need to add altimeter - Yaw - roll - pitch - accelerometer - cabin pressure - heart rate warning - tyre pressures - inside / outside ambient temp, humidity, dew point, barometric pressure - UV index - ambient light alert to turn on the headlights - sleepy driving sensor alert ( anything beyond a 15 second microsleep ) and most importantly - Twitter, Facebook and E9 forum Likeometers.