Everything: Take minecraft, add a real good game-engine and apply the rules of physics. Set shape,size,weight, center of gravity, and other things to every object created. Build a bridge and see if it holds! Different game-modes such as creation, "law-breaker", and of course some kind of adventure mode.
3 years, 3 weeks, 4 days, 4 hours, 49 minutes ago by PerMartin