It'd be nice if we were able to stack half-blocks of different mediums together, such as stone steps on top of wood, sandstone, brick, etc. For those of us that like to make realistic multistory homes, having a ceiling that doesn't automatically double as the upper level's floor would be a neat option. Making them two blocks thick adds an ugly factor to anything that's trying to look realistic.
Eighth Blocks (suggestion, mod) can do this as well as allowing a variety of shapes, such as sideways half-blocks and corner stairs.
Mostly moved on. May check back a few times a year.
Someone posted this picture stating it (mixed slabs, not wool slabs) was done in a mod already as well so t's not like it'd be too difficult.
*my topic for this suggestion*
http://www.minecraftforum.net/topic/741910-mix-and-match-slabswool-slabs/page__p__9691333__fromsearch__1#entry9691333