The reason why minecraft is so popular is probably the simple idea for everyone to be able to build cool 3D objects, use them and show it to others. The other are the athmosphere and exploration.
There are a lot of possibilities to build up on that, Also by adding more mission-based objectives into it. Like: "Next Mission: inspect the cave in the north-east and fine the diamonds" Or: "Stay at position x,y for one night, and fight off the monsters"
