In the lead up to the release of Avatar: The Way of Water, there were a collection of reports that seemed to suggest that Vin Diesel would be appearing in an Avatar movie at some point. This came after the actor was spotted on the set, leading to a flurry of speculation. This seems to have come off incorrectly.
Because producer Jon Landau has spoken with Empire and revealed that Diesel won't be in any of the Avatar sequels. The star was simply on set for a look around and for a quick visit, and as Landau puts it: "people took that out of context."
This is a bit of change in direction, as there have been multiple occasions where Diesel has directly been quoted saying he is 'doing Avatar' or that he will be working with James Cameron at some point. This could be alluding to something else, but considering Cameron seems to be completely committed to Avatar for the foreseeable future, what exactly is unclear.