Some sort of diaphragm shutter could prevent dust from entering the camera during lens changing.
I keep wondering why they do not make a TC with zero power. That does not change magnification or degrade lens quality. Then we could leave the 'plain glass' on the camera and change lenses without fear of dust on sensor. We could just blow away, wipe away the dust from the plain glass.
Nice idea ya? I wish I could manufacture this and sell! lol
get a sigma, they have a dust screen in front of the mirror.