If you want to make a separate window manager, it is likely that the result is going to be incompatible with most existing clients. But this seems par for the course for almost every Wayland extension so far (see Gnome...).
And beside gnome and kde there is a third big group of wayland compositors based on wlroots which does support some form of common set of extensions not in core, effectively making you write only the wm part.
Because this prevents creation of a separate window manager program. You can do all of this with the core protocol in X11. You can't do any of it in Wayland almost by design. The entire idea of a client getting simultaneous access to the objects of another client is not in the protocol. You cannot render in another process surface. You cannot even iterate over its windows! The role of the compositor is hardcoded in the protocol design.
The closest thing you can do is to write some type of modular compositor which speaks _yet another_ protocol with the window manager module (or even exists within the same process). But this is not like in X11, where you plug new things (e.g. a new WM, a new pager) without having to restart the X server, much less having to change it.