In the category "because HDNitehawk just bought a RF 600" Canon has just announced their RF 800 f/5.6 and RF 1200 f/8.
And they do accept teleconverters.
To help with any type of sting their Rick, the prices are $17,000 and $20,000, respectively.
Introducing the Canon RF800mm F5.6 L IS USM with Explorer of Light Charles Glatzer - YouTube
Introducing the Canon RF1200mm F8 L IS USM with Peter Read Miller - YouTube
Most eyepopping stats, other than the 6.9 lb (RF 600 is 6.8 lbs) and 7.4 lb weight, respectively. The RF 800 at 17 inches long, is shorter than the RF 600 at 18.6 inches long.
But, surprising to me in the other direction, the MTF of the RF 600 plus extenders really holds its own.