Here's what I would do if I were Canon:
* The new 1.6x crop sensor body uses the existing
R mount and for sake of argument let's assume it will be called a
RS camera.
* It can use any RF (FF) lens, using the 1.6x crop of the image circle.
* It can use any EF (FF) lens, using various EF-to-RF adaptors and a 1.6x crop of the image circle.
* It can use any new R (1.6x crop) lenses they might make, and for sake of argument let's say they're called
RS lenses.
* It can use any EF-S (1.6x crop) lens, using new various new EF-S-to-RS adaptors.
*
IF they can make a EF-M-to-RS-teleconverter (which
may or may not be possible as the teleconverter must extend the flange distance of the EF-M lens by 2mm + depth of converter metal mount - but hey - they
are Canon) then it can use any EF-M lens with a new crop factor.
* The RF (FF) body can already use any EF (FF) lens with various EF-to-RF converters.
* The RF (FF) body can use new RS lenses in 1.6x crop mode.
In addition (in case anybody wanted to do it):
* A RF (FF) body could use any EF-S lens in crop mode with new adapter.
*
IF EF-M-to-RS-teleconverter works, the RF (FF) body could use any EF-M lens with a new crop factor.
Hence their future of FF is RF, and the future of 1.6x crop is RS, and they're all interchangeable on the R mount. Everybody, y'all bring your existing lenses and join the
R party - All hail Canon!
If I made some glaring mistake, please let me know. I'm not a Canon expert, and am only thinking "outside the box" and I suppose it's possible I'm so far outside that I "fell off the table"