A prototype of a Canon EF 600mm f/4 DO BR lens is showing at the Canon EXPO. We’ve seen patents for such a lens in the past, along with a 800mm DO.
It definitely looks compact and I’d love to see the 800mm replacement get the DO treatment.
Below is an image of the development of the 600mm f/4 DO BR, notice the red ring? This is definitely a prototype to show the potential size of such a lens.