There is nothing stopping a Wayland compositor from exposing an interface that would allow for a choice of “window manager”. In fact, wlroots could almost count as such a compositor - it implements the bulk of a compositor, but none of the bits of a window manager. Of course, Plasma and Gnome also allow window managers to be integrated as plugins, but I presume that is not what you want.
It is not like the X window manager idea is impeccable either: To name one thing, picom or other compositors could display much nicer and context aware animations if only the window manager interface was not like it is.