Like many, I have my own ideas for adding something cool to minecraft.
My idea is to add both limestone and marble blocks

Marble -
Marble is commonly found in underwater caves, or deep underground.
It can be used as a common block, and has about the same value as mossy cobblestone
When broken, creates marble block.

Limestone -
About as common as stone, although there is more stone.
Found in huge pockets that sometimes extend aboveground.
Mix between green wool, sandstone, and stone. (colorwise)
When broken, creates 2-4 limestone shards, which can be used to craft limestone (4 shards = 1 block)