Canon is pretty generous with its "all new" description. When applied to 18mp sensors and previous RF superteles, we heard "all new" a few times, which turned out to be a squinting interpretation. I hope we do get some all new designs in the literal meaning. Like Neuro, I'm still hefting a 600mm II, waiting for something to justify the price differential. Because they refreshed the 400, 600, 800 troika recently with just some refurbished remounts and internal teleconverters, plus some IQ and AF fairy dust, I too expect the 300 f/2.8 or 500 f/4 to pop out next. Not that my expectations have been a good predictor of much.
The topic of the 200-500 1.4xTC rumor is interesting. I know me well enough that I'd be at 700mm the whole time with that, so if it added a lot of weight to get the zoom range, I'm not sure I'd bite. I'm sure the price differential between my 100-500 and this new lens would be around 10k, which I wouldn't be able to justify as a swap.