Canon Patent Application: Super Telephotos with built in teleconverters

This patent application describes super telephoto lenses, with the ability to flip in a built-in teleconverter.  This sounds very similar to the 200-400mm lens.

A main lens system having an aperture stop and a converter outside the optical path of the main lens system on the image side of the aperture stop of the main lens system and disposed so as to be removable in the optical path of the main lens system An optical system, 
wherein the converter is capable of changing a focal length of the main lens system when inserted in the main lens system, the 
converter has a plurality of lens elements, and the converter The lens element disposed closest to the object side has a negative refractive power, the focal length of the main lens system is fn, the total lens length of the main lens system is TDn, the converter is inserted in the optical path of the main lens system When TDi is the total lens length of the optical system when worn

As with all patent applications, this is an area of research for Canon.  Whether or not a product will come of it is certainly debatable.

Japan Patent Application 2018-180209