Keith over at Northlight-Images has uncovered a patent that showcases some of the capabilities of the RF mount as related to accessories such as extenders, converters and whatever else Canon can come up with.
As you may know, one of the biggest advancements in the RF mount over the EF mount is the high-speed communication capabilities, and it's like we've only scratched the surface as to what is possible for the RF mount.
We have heard some rumblings that RF mount extenders are coming, and I wouldn't be surprised to see an EF to RF teleconverter with a control ring for super-telephoto use.
I also believe some RF 70-200mm f/2.8L IS USM photographers would like to see some RF mount teleconverters as well.
The image below will give you some idea of how things flow between the camera, accessory, and lens. (Click for larger)