I see the RP as entry level full frame, the R as general purpose full frame, the R6 as general event/wedding shooters specific, the R5 as higher end general purpose full frame, and the R3 as high end event/sports full frame.
A ~$1500-$1800 R replacement would be great. ~30MP would be nice, dual card slots wouldn't be necessary, but nice to have, definitely the same processor as the R3, R5, R6. Basically make a nice camera that many of us working photogs would use when shooting tethered in a studio doing standard issue portrait/headshot/fashion/product stuff. The R3 and R6 are awesome on-location kings, and the R5 is awesome for tethered studio work, but not everybody wants or needs to drop that kind of cheddar on 2 or 3 bodies for every day tethered studio work. The R6 can be made to work, but for $2500, you're paying for a lot of features that you never use in the studio, and it's not quite enough resolution for retouching, skin smoothing, etc. The R, while long in the tooth, is nearly the perfect combination of resolution and features for every day general purpose tethered studio work. Like I said, the R5 is great, but when you're pretty much shooting every day, file sizes are something to consider. You want enough resolution to ease retouching, but not so much that you're constantly dealing with storage.