I've been following a blog called procworld it is a project by a single Canadian to create a voxel based game engine that procedurally generates the entire world. Kind of like minecraft, but each voxel can be manipulated. Today he just announced a partnership with a game called StarForge, and while its all still in early development, the results excite me quite a lot!
Picture it this way, what would happen if you had a world where you can explore it infinitely (digging into the planet, exploring the surface of the planet, or even blasting off into space? Ditch the conventional notion that planets are round, and you can have great fun! Throw onto that borderlands esque weapons, and the ability to build/cut/modify every terrain feature you can see.
Check out the links for more info.