In real camera we have auto exposure feature. I know that this is hard to implement in render engine mainly during animation where light changing during time but it can be simulate by user using exposure picker with the same way as DOF picker.
If we would have settings for minimal and maximal exposure and user pick render area RGB value ( from HSV mode ) this brightness value ( from 0-255 ) should be used to set average exposure from given exposure range. In this way we can simulate real camera auto exposure and animate it.