Describe the feature

It would be nice if the slab corners could be connected. That would allow you to make any combination of 1/8th blocks, and would allow things like artificial stair corners. It would also allow customization of the texture of each 8th of the block.

Combining arbitrary shapes or arbitrary amounts of shapes in a single block is not feasible and will not be added to FramedBlocks.
If you need to be able to combine an arbitrary amount of textures and shapes in a single block, then a mod like Chisels & Bits is better suited for those cases.