Are you sitting down? Sensor size is history.
An optical reducer is something I have long thought was possible on a DSLR and wondered why nobody had made one. If your sensor is smaller than full frame, shrink the image that the lens throws to fit over it. That is the principal behind the Metabones Speed Booster which essentially gives you the full frame look and a brighter image all at once on your Sony E-mount camera.
Crop factors are a thing of the past.