We're told that Canon will update the Cinema EOS C300 Mark II in 2019, with NAB 2019 in April being the most logical place to showcase the new camera.
We're told again that the Cinema EOS C300 Mark III (as we're going to call it) will launch as a 4K camera, but that there will be an 8K “upgrade” option. What that would entail is unknown at this time.
Canon has admitted they were slow to adopt 4K and that they wouldn't make the same mistake again. That said, is 8K really ready for mass adoption? Probably not. But a likely high cost, low yield upgrade on a popular camera could be the way to be one of the first few companies with an 8K option for the people that want it.
The same source did say that an RF mount Cinema EOS camera was in development, but likely won't be part of the 2019 roadmap. Even though the EF/RF adapters are superb, it probably makes more sense to wait until there are more native RF mount lenses.