The 300-600 is a possibility, but I'de not hold my breath waiting, I could only imagine a quite small market, but when you consider the size of the 200-400, I wouldn't imagine the 300-600 being much bigger in size than the current 600, slightly heavier, as is the case of the 200-400 over the 400f/2.8 II. Price ?? I'de imagine a similar premium that we have on the 400f/2.8 to the 200-400f/4, So around 20% more expensive than the current 600f/4 II.
The 600f/4 (1.4x) I see as a definite, Nikon are offering their own Long Lenses now with a dedicated Converter as part of the package, albeit it's similar to what is currently being offered but tuned to the particular Lens, which I feel is a good idea.
Having the 200-400f/4 (1.4x) I immediately saw the possibilities for Canon with the built in 1.4x Converter especially if it was offered as a "Build to Order" option on the 400/500/600/800, that way they could keep the standard no converter Models as they currently do, and the current prices, but have the more expensive "Option" for those that see the benefit and can afford it of the Built in Converter & don't mind waiting 6 months for a Special Build Option.
If they offer it up I'm certainly offloading my current 600f/4 II & jumping into the 600 with the Built in Converter, great idea.
Need Gura Gear to make a slightly longer Battaflea 32L Bag then, currently the 600f/4 fits one side, just (but not with the 1.4x Attached), and the 200-400f/4 the other side.