I really want to see the in-field performance of the 400 f/4 DO II under different lighting conditions. If Canon has solved the DO weakness of poor contrast and flaring, and knows how to make super-sharp DO lenses, then we should expect to see a 500 or 600 f/4 DO someday - maybe 5.5 to 6 pounds, rather than the 7.0 and 8.5 pounds the version II conventional 500 and 600 f/4 weigh. Yeah, I know - unicorns.
I'm sure its possible, it depends on marketing. Are there enough buyers in the market to go for a 500mm, a 600mm or a 800mm DO? A lot of people recently invested a big chunk of money in the new 500mm and 600mm lenses, and are not likely to change to a DO version, so maybe the 800mm is the next DO lens??
I'm surprised you're so sceptical about DO with how much people raved about the weight difference between the Mk1 and Mk2 Big Whites.
My bet is all Mk3 Supertelephoto lenses will be DO, there really isn't much else they can improve otherwise.
I think by 2020 the 500 and 600 dinosaur primes could be replaced by a single 400 - 600/4 DO zoom unit.
Canon are getting the hang of DO now so this would be a natural developement - two for one! Primes are going to be replaced by zooms.