While there’s long been an X.Org Wiki page with some pipe dreams for X12 as the successor to the X11 protocol, don’t bet on it ever happening.
Plans for X12 are occasionally talked about but there’s been no actual work towards this milestone. As most Phoronix readers know, a majority of the upstream “X.Org developers” are now also contributing to Wayland or focusing upon parts of the stack that carry forward — such as Mesa, DRM, libinput, etc. However, in what seems like comic relief, every few years there’s someone coming out with bold claims or suggestions for the next-generation X.