It took but a few minutes to get my first inquiry about Canon's plans for an EF 200-600mm lens once the announcement of the Sony FE 200-600 F5.6-6.3 G OSS for their mirrorless line.
As we sit here, I have doubts that we're ever going to see such a lens for the EF mount. The RF mount? That is probably more likely. However, I doubt the EF design would translate well over to the RF in a way that Canon would be happy with, so it could be a case of starting from square one with the design of an RF 200-600mm f/4.5-5.6 IS.
The image above is for the Canon EF 100-400mm f/4.5-5.6L IS II USM