Wait, I got an idea for normal & upside down slabs.
Will we be able to slam 2 different-material slab into one block?
If so, that means we can have better floors/ceilings now!
The developers would probably have to re-code and re-skin current and future adaptations of the slab blocks to comply with inter-material double slabs.
So you figure we have 6 different types of slab blocks.
Multiply that dimension by the number of different slab types there can be in any one tile, you get around 12.
Unless I misunderstand how Minecraft/Java works, that means we need 12 new skins for these inter-material blocks and their respective faces, and maybe some modifications to the blocks' classes and their interfaces to render these adaptations.
Not really worth the work.