The fact that Diplomacy does not have a feature to suggest/demand that another Civ make Peace with another, whether it is a Civ or CS, is horrible. This coincides with Civ V where City States contribute nothing on the international scene to create tension among the competing Civilizations. Or even throw possible Diplomatic curve balls at players. For example City State's having their own set of foreign policy goals towards other City State's nearby. One DoW on other that is allied with another Civ. All of a sudden you could have the potential of a small war becoming a big one. Or even creating a proxy war. None of that is possible.