Use the Weston full-screen shell. No need to invent a new Wayland compositor for that, and certainly no reason to have multi-display-system abstraction like Mir seems to have.
You would still have to fork it though if you need to add missing things, like how now Gnome and KDE implement in different ways how to grab screenshots. The fact that smaller DEs are creating their own libraries and not use Weston indicate to me that Weston is just a playground for Wayland where they test their ideas and experiment and it is not designed to be used or extended, I may be wrong though, there are some blog posts from the authors of Sway and others that work on adding Wayland support to different DEs. Why do you think Sway, Gnome, KDE and others can implement what they want but Canonical or volunteers can implement wayland on top of Mir but have to use Weston or Mutter ? Do you think everyone should Weston or Mutter?