My idea for Minecraft is as follows:

Each solid block would have a slab and stairs variant but rather than clogging up the menu there would be something else. If you left-click the block, a mini menu would appear with the block and its variations. You could then pick the block that you want. (see image) This would also apply to pre-existing slabs and stairs and would make the menu much easier to navigate. No more would the menus be confusing because of stairs and slabs in random places. It would be perfectly organised.

Rather than making a texture for every slap they could just be half of the parent block. This goes for stairs as well. They could completely streamline the system of making these special blocks. Stone slabs could be changed to tiled slabs and stone slabs could have the texture of stone.

This would help builders who want detail and a consistent pallet, rather than having to improvise and use other blocks. I would love to use this on almost every project that I will ever undergo and my end products would be much better. I know that building my towers, mansions etc. would have been much easier with these blocks. Take notes Microsoft.

There would be grass and log slabs/stairs as well. There could be options when making a new world to have the terrain form with these 'natural slabs' to make the hills and mountains less tedious to climb. They could even make these new slabs and stairs for glass and in creative there could be ore slabs and stairs. The options are endless!

As PRXMbuilds said, there could be walls and carpets for everything too. These would also be added to the expanded menu (see image) and would also be very helpful, though that wasn't what I originally imagined.