I think that rumor turned into the 100-300. I don't see Canon launching a 300 now, or at least not any time soon. They would potentially piss off a lot of 100-300 buyers, and the price really wouldn't be much lower anyway. There was a video that Canon made about the development of the 100-300 and it gave me the impression that it is the new 300.
As for the 500, Canon has really filled out the RF lineup of 400/600/800 so I personally don't see a 500 coming but I'm definitely not tied into the latest developments. There's a 400/2.8 and a 600/4. I dunno, maybe a 500/4 could fit in there but also Canon could make it a 500/2.8 for $16,000.