I've been thinking about that, Canon extending the barrel (with filter slot!), updating electronics and motors and adding their newest coatings.
My suspicion is that Canon hasn't show these in public is because it would look lazy and from a spec sheet PoV nothing would have changed compared to the EF versions. Why would I stop using my EF85mm f/1.8 + filter adapter and switch to the RF version?
The only downside I've learned about so far using EF lenses is the R's EVF only allows High-speed Display when using an RF lens. (Page 149, EOS R user manual) Frankly, even with high-speed display, I can't see the R as an action camera. Hopefully things will improve in a future body, but I wonder if Canon will change this particular EVF restriction.