Canon recently has filed two new diffractive optics lenses in Japan. These two lenses are 300mm f/2.8 DO & 400mm f/2.8 DO. According to patents, these two lenses are probably RF mount lenses. We may see these lenses announced in 2020 with EOS R verison EOS-1D X Mark II.
Japan Patent Application 2018-189878
Focal distance: 392.58
F number:2.91
A half field angle: 3.15
Image height: 21.64
Length of the lens: 363.53
BF: 59.15
Focal distance: 293.40
F number: 2.91
a half field angle: 4.22
Image height: 21.64
length of the lens: 274.00
BF: 59.35
via: CR