I've noticed that there are some flaws in the diplomacy system. For example in my last game English & Persians were constantly insulting me & I was at good terms with Russia & Babylon. After sometime I conquered the only city of English & took 2 cities of Persians leaving them with 1 city. Now Catherine started calling me bloodthirsty.
That was revenge actually. If Casus Belli System is introduced, such problems can be solved. It will also help in explaining you why nation X declared war on you etc. You could get a casus belli against someone who insults you, who is too weak, grabbing land near your empire etc. So what do you people think about it ? 

