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, 6 months, 2 weeks, 3 days, 19 hours, 29 minutes ago by PerMartin