Japan Patent Application 2018-180203 shows off some optical formulas for superzooms with both EF and RF mounts.
EF mount optical formulas included in this patent:
EF 35-350mm f/3.5-5.6
EF 35-400mm f/4.5-5.6
RF mount optical formulas included in this patent:
RF 25-300mm f/3.5-5.6
RF 30-350mm f/2.8-4.5
A superzoom with some speed would definitely be a welcomed addition to the RF mount and the EOS R system.