MICROWORLD: What sort of content would you put into the game? What would be the underlying principles for the game?

MICROWORLD:

What sort of content would you put into the game?  What would be the underlying principles for the game?   I think a project like this might take a very large team and a good amount of time to accomplish.   I wonder if you can think of simulations or games which already exist of which you might take advantage.   Microworlds have the advantage of allowing students to find their own knowledge by exploring with the game.   This presents certain challenges in defining the role of the teacher.   How much involvement should the teacher have in the game?