Keep the same body and controls. Swap in the sensor, focus system, and hot shoe from the R3. Retain compatibility with the same battery grip as the R6. That would be enough if the goal is to maximize the performance while keeping the price attractive.
Of course, I would also appreciate a higher performance EVF and screen, but not having those would not be a deal-breaker for me. I'd still upgrade ASAP.
I'd love to see this. Just take the R3 sensor as is and put it in the body of an R6 with the current R6 body and change the hotshoe over to the new accessory shoe, with maybe minor tweaks to the firmware for functionality along with a bit better heat management for the video shooters. Keep the price the same. We get a bit more resolution, way more dynamic range, better video performance, but otherwise it's the same "entry level" event, concert, sports camera.
I'd drop the R and just put the R sensor with maybe a couple tweaks into the RP with maybe minor tweaks for an RP update if they want to retain the entry level full frame. If not, then drop the RP and figure out where the R users are going to go, or release a full frame replacement (R8, who knows) in the $1500 to $2000 range with the R5 sensor and the R5 gets a new sensor in the ~60MP range, then an R5r also gets released with at least 12000x8000 pixels, though at this stage, anything more than 6000x4000 really is to address specific usage scenarios because 20-30MP covers the vast majority of resolution needs that most of us actually need. I know we all want more resolution, but really, unless you're printing really huge (almost nobody), or doing extensive post work (again, niche use), for general use photography, how much more resolution do you really need? I'd almost rather see Canon standardize on a handful of resolutions and work to optimize them for the best performance, then differentiate their camera line with features and functionality/form factor. Having a whole pile of different sensors is expensive to manufacture, get it down to 2, maybe 3 full frame and at most two crop sensors. Right now they have no less than 5 different full frame sensors (in terms of resolution), and at least 4 or 5 unique crop sensors. That's a lot of chip making overhead.