I haven't used linux as my primary OS for many years now. If not X11 or Wayland, what other alternative? Is there some other relevant development going on?
There's Arcan: https://github.com/letoram/arcan . In the words of the developer, it is a "multimedia engine" (a combination of display server, game engine and multimedia framework).
It is an interesting project and the YouTube presentation is pretty convincing: https://m.youtube.com/watch?v=07nqZIFRDJg
A great example of what a motivated and disciplined developer can achieve.