According to a reliable rumor source (thank you!) Canon is testing prototypes of the following DO super telephotos;
RF 400mm f/3.5 DO IS USM
RF 400mm f/4 DO IS USM
RF 500mm f/4.5 DO IS USM
RF 500mm f/5 DO IS USM
RF 800mm f/9.5 DO IS USM
According to the source, only one of these is expected to make it out of the prototype and into production. The 500mm lenses have had the best results so far, with one of them currently in the wild.
While we didn’t have an RF 500mm DO IS USM on our roadmap, with current supply chain delays Canon may be forced to do something out of order with these lenses.
Interestingly, the source states that Canon hopes to announce this lens either with an actual announcement or development announcement in conjunction with the R1 announcement. The source mentions that production delays may force Canon to do just a development announcement for this lens this year. The source is certainly implying that the R1 announcement is expected later on this year.