I imagine 700/29 will be the size for the foreseeable future. Single diameter and disc brakes are probably a rim manufacturers dream.
While I’m not up on UCI rules, there appears to be no specific mountain bike diameter constraints for mountain bike races. I have read that 1.3.018 is in conflict with 29er diameters exceeding 70cm, “including the tire,” but I doubt a conflict exists. However, I can see some mtb specific language that would prevent a company from competing on a larger rim. And no company would develop a size that would get banned.
John