i cannot see Canon adding f/6.3 lenses to EF portfolio all of a sudden, when they have avoided this for 30 years. For good reason: AF limitations beyond f/5.6 on most EOS DSLRs.
i could imagine f/6.3 lenses for R mount, because mirrorfree EOS R series cameras will/should be able to AF to f/11.
Whether or not f/6.3 is a "desirable open aperture" at 60 or 70mm focal length is for each potential user to decide. I don't think the difference in size, weight and price between a 16-60/3.5-6.3 and a f/3.5-5.6 are so huge to make it worthwile.
this said, i currently do own and use one Canon f/6.3 lens, EF-M 55-200 and have also been considering EF-M 18-150/3.5-6.3. Nothing else available from Canon and size/weight, focal length range, price make it "borderline acceptable" to me. Although i would really prefer f/5.6, especially on APS-C.