the ability to dig down into the eart and create ant-farm like mazes, creating caverns and so on.
this could allow the discovery of dinosaur bones, buried treasure, glittering gems and so on - as well as for the creation of player-generated spaces!
and maybe if the tunneling becomes too extensive the whole level could collapse! CRASH!
or maybe if you dig too deep you'd strike lava and it would fill up all the tunnels and then harden!