Canon's Kengo Iezuka suggested recently in an interview with Mynavi News that Canon is considering more lenses like the RF 600mm f/11 STM and RF 800mm f/11 STM super-telephoto lenses. He notes that these types of lenses weren't possible on DSLRs because of the autofocus technological differences between a mirrorless and a DSLR.
“This lens has a darker F11 opening value, but it can be said that it was born because of the EOS R, which can use high-speed and high-precision AF even at such an opening value.”
Mr. Iezuka was asked specifically about the possibility of an RF 400mm f/8 STM to complete a trilogy of super small super-telephoto lenses. It sounded like something he wanted to see by saying “It would be nice to have it completed”.
I think the sales numbers of the RF 600 f/11 STM and RF 800 f/11 STM will determine if Canon continues with these lenses, as they did take a bit of a gamble investing development dollars into something we haven't really seen before.
You can read the full interview here.