Offer an HVAC button to vent only (no heating or cooling, but fan operates).
At times, one wishes to simply flow air through the car without heating or cooling it, and without opening windows or roof, for example when traveling at highway speeds in the sun that warms the car under otherwise cool conditions. It would be more convenient to have a single soft button that sets the HVAC system to automatically ventilate only. As presently configured, this requires one to set Auto to off, AC to off, and then set recirculate on.
A single button could configure all of these for maximum ventilation, with the auto fan control that responds to the discrepancy between interior temperature, exterior temperature, and the target temperature. The logic for the fan could be overridden as usual, but would go something like this: If the exterior temperature is lower than both the interior temperature and the target temperature, the fan would be proportional to the discrepancy between interior and target temperatures. That is the fan would blow more when the car is warmer than desired, and there is a source of sufficiently cool air outside the car. Otherwise, the fan would be off unless overridden.