I think it would be a great addition to the diplomacy screen to have private chat between the two players, or some sort of text message system (only in MP obviously). This would be good as it would considerably expand diplomacy right now, e.g. you could ask an Amoeba to tell you of their enemies actions, or of the best nearby systems to colonize for a dust or some techs (if you did it in all-talk chat you would tell everyone else!)