The R3 is more than sufficient to fulfil the needs of most professionals. But manufacturers are always striving to provide more. So what are the few remaining areas where improvements would be beneficial, and where does that leave the R1?
Global shutter? - possibly, but ultra fast readouts would reduce rolling shutter to the level where it became virtually undetectable, so global shutter may not be necessary.
Canon patented, 2-3 years ago now, a sensor design that has global shutter, built-in ND, and can almost double dynamic range. It's a compelling package that would get my interest even though R5 build quality and MP is enough for me.