Canon filed a new patent “400mm F5.6 reflex-mirror lens with an electrochromic ND filter for APS-C DSLR camera”. Since the patent was filed so we can expect its arrival somewhere at any point of 2018. No further detail has been included yet.
Specification and lens arrangement of embodiment 1
Focal length: 400 mm
F No.: 5.6
Image height: 13.66 mm
Back focus: 33.07 mm
Lens length: 247.73 mm
On the other hand, in the past rumor from NorthLightimages it was suggested that there are a cluster of lenses specifically the Supertelephoto lenses to be announced next year. They say lenses design from 300/2.8 to 800/5.6 are in the Horizon.
You should know that Canon has no dedicated 400mm Telephoto prime lens for APS-C DSLR camera that this lens must be the anticipated lens.