The shape idea is for the purpose of making custom shapes. I wish there was an easy way to place them vertically....shift or CTRL+click maybe?

I know with mods like "redstone paste" they use a type of green "targeting/selecting" graph that shows the 4 sides and the center to place the redstone paste (I think forge microblocks also does this) that way you can place a slab on either of the 4 sides flat or right up against whatever your placing it up to.
It's hard to describe through typing (at least for me), but that system is what my thoughts have been for vertical slabs and why i've been wondering why they didn't add that type of system yet seeing as it's worked so well for other mods.

I like the idea but the issue is that slabs would now take longer to build (vertical), it's something I've never seen before though.

I don't understand how they would work with the 1x1 metre voxel 'basis' for the game though; only 1 block can inhabit 1 square at a time. Although I can see what you're proposing working, that said... In the same way that a single slab is actually just one block while 2 stacked slabs are also 1 block, perhaps all the possible permutations of these quarter blocks could be individual 'blocks' themselves?

Alternatively, just have blocks which are 50% of the block space (just like slabs) and which can be placed 'directionally' like stars so they can adhere to any given face of the space they inhabit. So that would be possibilities for: one adhering to the northern side, the eastern side, the southern side, the western side... And then two options for combination blocks: the seam facing north-south or east-west. So it would only require 6 block 'forms'. Stairs have at least 15 or so block forms, so I don't see why it wouldn't be possible!