Alright, so I've put some thought into this overnight.
I really like the shattered world idea. I'm thinking that the playable world could be divided into two parts: an upper continent, and a lower continent. The upper continent has been obliterated, and floats seamlessly above the lower continent; a much larger, stable landmass. The players spawn somewhere in the upper continent, in a largely destroyed city with the obligatory flaming barrels. The city and its underground portions are free to explore by horse and hang glider (refer to that picture above for a sense of what I'm getting at). The rest of the upper continent is largely uncharted, and consists of other floating ruins and smashed landforms.
The players adopt a Skyward Sword feel to the lower continent - it's hellhole where people go to die. Much more varied environments, oceans, and more bandits. Monsters, etc.
The occupants of both continents have made tools out of whatever materials that they had at hand, and again, have made use of horses and hang gliders rather than space ships and laser torcreepes.
I had an idea for premade content to build with, such as maybe tent and fire bricks. Fire could keep out land monsters or something - a good reason to hunker down for the night. idk. People have said that they wantd to build establishments and whatnot, so this sseems like it might keep creativity way down - a negative. This part still needs more thought. The underside of the lower continent could be one custommade brick - if that's possible.
It all sounds really ambitious. :P