I do not intend this post as anything but an observation and an idea as to where the game could go.
How to Host a Dungeon (HtHaD) is a pen and paper, subterranean civilization 'simulator.' (links at the bottom)
Beginning of the game involves placing hidden treasures (gem deposits) and hidden dangers (lava and water rivers, big baddie lairs) underground. Second stage involves simulating a great subterranean civilization (dwarves, dark elves, etc.) that mines about, building and collecting treasures into secret vaults and such until a great catastrophe wipes them out. The final stage represents when surface dwellers start to dig down and discovers these ancient ruins and rooms. The original intent was that after simulating the third stage for a bit, it would become the basis for a Dungeons and Dragons like campaign world.
Minecraft right now is like this without the second stage. The player is the surface dweller who is finding the hidden treasures (diamond and gold deposits) and dangers (mob spawners and lava flows) beneath the surface. Imagine if you could also find hidden subterranean cities now occupied with the baddies that wiped out the prior tenants, and fighting past them lead to their vaults filled with their treasures.