Every plea I see here for a "casus belli" system basically amounts to "It's unfair that the AI stops liking me when I start killing people."
I still can't see the sense in it.
It's certainly not my case, I play peacefully 80% of the time and for the most part I'm okay with the warmonger penalties as I don't see Civ as a game of unbridled Hitler-like warmongering - at least not without serious obstacles in your economy/happiness or science.
It's the diplomacy for anything short of war that I find lackluster and bland. A few steps were made to make the diplo system more interesting with Civ V but I'd like to get even better, more complex and realistic in future versions.
By "casus belli" I'm not quite referring to warmonger penalties but largely referring to a modulation by era and a greater diversity through the game. E.g.: religion should be a greater cause of tensions and aggression before Industrial. Concretely, practicing religious tolerance (letting traders to your city practice and thus perhaps spread their religion) could make the AI more likely to do the same. However, killing missionaries (which should be possible without declaring war, if at a diplo price) or using inquisitors actively (passively, it could stop influence coming from trade route: ie: traders are forbidden to practice their religion while in your city, negating all religious pressure from the TR) could lead to retaliation (the AI asks for a compensation, cancel your trade routes to them or purge your religion from its cities etc.) - and escalation could lead to crusade, in which other civs following the same religion are likely to either join depending on their diplo status with the offender, or else take measures like stopping trade or purging your religion from their cities.
Greed over wonders is a really silly cause for war in the modern era, but sitting on half the world's oil and refusing to trade it to some civs though...
Expansion is a rather poor casus belli in the Ancient era, when the "empires" consist of 2-3 cities, still "unconnected" and thus largely city-states united culturally only still, and half the landmass is still unsettled. It should be causes like greed for luxuries on the claimed landmass that excites tension (which could be alleviated by trading it to them), and greed for spots/cities that would let a civ reach other cities with Trade Routes, or access to the coast if a civ finds itself landlocked and unable to trade abroad and even explore. Once there is no room for expansion border conflicts should become virulent for a while (starting in classical in crammed areas, but mostly in Medieval and Renaissance), before gradually fading again by industrial (except for the Hitler-type leaders). It shouldn't be a cause of tension in Modern if civs are above "hostile" (you don't see much the US coveting the lands of Canada or Mexico...)
Warmongering penalties and what cause them should also be modulated by era, and reflect more their flavors.
