heres an idea, larger empires have lots of corruption and are harder to maintain, so you should have the option of splitting your empire, basically a "peaceful, premeditated civil war" lol. the example is obviously rome. though post WW2 europe works quite well too. this would work for decolonization as well as just creating safer more defensible borders.
also, diplomacy should be done with several civs (ie congress of vienna) so they can come to a concensus on what the new map should look like, who gets what area, and of course, what new civs are formed where (ie the netherlands after napoleon)
i like the multiple leader idea, but i thinkk it should be done by random chance. so you may at one point "lose your leader" and need to choose a new one. or multiple factions arise and you pick which leader to play as (as previously mentioned) this would allow different dynasties (ie china) and i think upon leader change you should not only start with a blank slate as far as diplomacy goes, but you should get to pick 2 new traits, though they could be the same.
for instance, china is mmilitaristic and expansionist. they go through a civil war, but since all areas were reconquered and they no longer need to expand or be militaristic, they choose commercial and industrious so they can rebuild their infrastructure after a long and bloody civil war.
heres another new idea, use leaders as units, like in regicide. only they have different stats/abilities. for instance, the warlike mongols have just invaded and taken control of your govt, so your new leader is now ghenghis khan, a strong mounted cav. unit with extra movement points or blitz or something. when he dies (through random number generated "event" or in combat) a new leader comes about, there is a possibility it will be another great cavaly king that you can use to conquer new regions, but it may also be a kublai, who stays in a city and adds happiness, commerce, science, increases production, or whatever else you assign him to. if it is a senator kind of leader, it can initiate propaganda like the spies from civ2 or reduce corruption in a region/city.
however, different govts have greater chances of different types of leaders. like in a democracy, you are more likely to get a senator type leader who will do things to improve the domestic area, wheres if you are a barbarian tribal govt, you will most likley get a warrior king (however you CAN get either, it is all by probabilities)
This way, when your leader dies (which he WILL DO) the game doestn end, but simply changes. you now can rethink your strategy. your military conquests havent been going well, so its time for a period of peace and domestic development, so you change your traits and are lucky enough to have a senator type leader to speed up this change. well you all get it... let me know what you think
