Is this the next evolution of the Canon EF 200-400mm f/4L IS USM 1.4x? Here is a patent showing a dual teleconverter Canon telephoto prime, an EF 400mm f/5.6.
What’s cool about the dual teleconverters, is one is a focal length reducer, so you get the EF 400mm f/5.6 and the built-in focal length reducer converter brings the lens to an EF 300mm f/4, or at the other end, a 560mm f/8 lens.
There has long been a desire for a replacement to the classic Canon EF 400mm f/5.6L, even if Canon just added image stabilization, but this is definitely a unique direction to take the lens on Canon’s part.
Japan Patent Application 2019-70689 summary:
Canon EF 400mm f/5.6
Focal length: 391.89mm
F number: 5.80
Angle of view: 3.16°
Image height: 21.64mm