How about changing coal deposits to diamond deposits for high level players? I see a couple of advantages with this. First, it would provide a new goal for high level players to work towards. Secondly, removing all or part of these players' coal deposits would encourage interaction among players since these high level players would need to get coal from other players.
So what would players do with these diamonds? I don't know... do I have to figure out everything around here??
Oh and btw, yes, I know that diamonds are not actually formed from coal. I figured that some of you would pull up my post and read it just so you could correct me on that... gotcha!