Possibly about this time next year something might be said. Shimano's last few generations have run on a four-year model cycle, with model years roughly six months ahead of calendar years... In XTR terms, that's M960 in 2002 for the '03 model year, M970 in 2006 for MY07, M980 in 2010 (MY11), M9000 in '14 (MY15) and M9100 in 2018 for MY19. So on that presumption, the next generation M9200 could be expected for release in mid-2022 for the 2023 model year.
PS: A few years ago Shimano suffered a fairly significant fire in a section of their primary Japanese plant, which produces their highest-level XTR and Dura-Ace bits, as well as some XT/Ultegra stuff. This led to considerable delays in the production & availability of the then-new XTR (M9100), and also pretty heavily fucked up supply of a lot of other decent-spec bits.