I think the slow and stable development of xfce is a good combo with rolling release. I think we get gtk3 xfce before we get fully functional wayland ecosystem. Then converting xfce to wayland might be just a matter of changing window manager.
I think that the fact that xfce is lighter than mate also works in its favor. This means that it suitable for wider range of machines, though the margin is narrower than compared with say, gnome or KDE.
But I think mate could be a good candidate for official edition too. I don't have much experience with it though. But it has fairly large user base because of mint and ubuntu mate, and is not the forefront of new technologies. Both of these should contribute in its stability.
I think for official edition, some important attributes are:
- full featurednes / completeness
- security (= points for wayland)
- not tied to one initsystem, because manjaro supports two
- lightness (manjaro has users without access to premium hardware)