This is a very simple idea, when the world is generated the procedural generation algorithm probably uses variables to determine the frequency of rocks, gras, the types of ground, etc. What if there were sliders to change these? That shouldn't be too difficult to implement and would add a lot more replay value, as you could do challenges like trying to play in a world with no ice generation, all water has to come from rain, or almost no grass generation so you have to create farms early on.