The Meaning of Life, the Universe, and Everything.
A very simple idea, non-solid blocks, such as fences, can occupy the same space as carpets and snow layers.
The very slight change would allow for a drastic improvement in building in snowy biomes or using carpets in builds. The way it would work is by, when you placed a block on a carpet or snow layer, rather than destroying it or floating above it the block is placed on top of the block underneath the carpet or snow layer.
This would mean that fences can easily be built in snowy biomes, flowers can be placed in the snow and patches of tall grass could peak out of the snow without showing the grass block underneath. Carpets could now also have tables and furniture placed on them, you wouldnt need to leave holes in your carpet for chests or beds or doors and decoration can be placed on the floor.
There also exists a huge opportunity for traps and secrets as redstone wires, preasure plates, buttons, trip wires, buttons and rails could be obscured by the carpet or snow layer.