My guess is Canon will keep the single digit number models for full-frame models. So here it goes:
- R100 for the APS-C if it is more like a slight above entry level camera OR
- R10 for the APS-C if it's spaced like a the 80d/ 90d series
- R9 for entry level --> basically just a sensor in a camera housing
- R8 for the new full frame below R6/5 --> R8 also shows some distance to them...
R7 will stay reserved as a placeholder in case Canon decides to offer a truly pro APS-C model...
I've always found it strange that in the xxD series, a high number (90D) is more advanced (and more recent) than a low number (60D), whereas with the xD series, a high number (6D) denotes a model less advanced than a low number (1D).
It's equally odd that ALL the full frame DSLRs had single digit designations, whereas the APS-C models had single (7D), double (80D), triple (750D), and quadruple (1000D) digit designations, plus weirdo alternative names - the 850D, Kiss X10i and Rebel T8i all being exactly the same model!
As for the RF mount cameras, my undoubtedly incorrect prediction is:
Hi end models, whether APS-C or FF, will all be single-digit (R1, R3, R5, R6, R7)
Economy/beginner models will all be double digit (R10, R50 etc).