I spent a week with an EOS RP and RF24-105f/4.0L two weeks ago, and I have to say that if there is any singular reason to dive into the R series, its the strength of the lenses. They're unbelievable (in the limited exposure I had). The single biggest negative would be the lack of cross compatibility from the RF lenses back to something else. The lack of an RF-EF adapter makes sense because of the shorter flange distance. An RF to EF-M adapter is a little tougher - the difference in flange distance is only 2mm, but that still seems doable. But there must be a reason why Canon doesn't want to (or can't) make RF lenses usable on EF-M mount cameras like they did with EF mount lenses. Personally, I think that they have to view the M series mirrorless market - which is still pretty strong in Asia - as a completely separate market from the DSLR and R series mirrorless markets, with very little crossover. I am certain that I am an anomaly in that regard.
But it got me to thinking that the rumors of an APS-C R body would be an interesting development in the R mirrorless architecture, as it might fill in a little bit of a gap. Especially now that they've added a "consumer-level" (or at least non-L) lens in the RF 24-240 f/4.0-6.3, it makes me wonder if a small APS-C R body might be a decent seller. As with some of the initial kit offerings with the RP, Canon has also shown that they're not afraid to fill in the lens gaps by including a lens adapter and an EF lens.
So on a rumor scale of negative 50, SWAG (Some Wild Ass Guess) is that an R series Rebel mirrorless camera would be a suitable (and potentially logical) addition to the R series lineup, and potentially a good seller.