Drilling holes could make many parts of minecraft innovative. Thinking of things from water seeping through, to things affected by gravity, such as sand, gravel, etc, have the ability to force through the small hole and form into the full block eventually, or maybe into a custom half block. Water advancement would be imperative I'm afraid, as the ability to "Fill" an area as the water dripped through would be really cool, I think. Or, you could simply duplicate the code of putting down water buckets until every empty block in an area was filled with 1 bucket of water each as if being poured.
Drilling holes