Another Rainbow Six game is said to be in development, possibly going by the codename Atom. This comes from well-known Rainbow Six leaker Shiny, who says that Rainbow Six Atom is in development at Ubisoft now.
The leaker doesn't have any further details to share about the new game, including whether it will be a main release or some kind of mobile spin-off. Right now, Ubisoft hasn't confirmed that Rainbow Six Atom exists, so even if it does, we'll have to wait for an official announcement.
Even if this leak proves to be accurate, we'll have to wait and see what kind of Rainbow Six game this ends up being. The main game right now is Rainbow Six Siege, which has been going strong since 2015. It would be huge for Ubisoft to try and drop a successor on us, so it seems more likely that this would be some kind of spin-off. We've had that before, with Rainbow Six Extraction launching in 2022.
As we can see in the replies, many are hoping this isn't a mobile game, at the very least. That seems unlikely too, since we already know that Rainbow Six Mobile is launching...