Another topic we should look at is wayland support. GNOME and KDE are forerunners in that respect. XFCE is still on the way to porting to GTK3 which it needs for any basic wayland support. GTK4 is being planned right now. So they are way behind.
Wayland means getting rid of a huge security hole that is X. At least in Stretch X doesn’t run as root anymore but still.