I have some thoughts on the Cerinrim and vertical slabs.
The Cerinrim, although a popular planned faction, are now possibly not going to be added, probably due to alignment overlap. To elaborate, the Cerinrim are enemies of every faction in Far Harad, which poses some alignment problems. For example, if you were allied to the Taurithrim but killed some Cerinrim in a Mowaith area of influence, then you would gain alignment with the Morwaith, or vice versa.
I think that the lore should be rewritten to make the Cerinrim somehow the friends of the Morwaith or Taurethrim to avoid this issue.
Vertical slabs are going to be added in the next renewed snapshot, hence the long update time. However, vertical slabs are probably going to be added to Minecraft in the future, so it seem that some of the vanilla block vertical slabs will have to be deleted if a update adds them.