Just an idea to implement civil wars using civics:
1) WITH ANNOUNCED RULES:
Changing civics like Religion or Slavery could lead to revolt in a country where there are more than 1 religion or where some cities use/have used lately slaves.
Changing from monarchy to republic would lead to royalist´ cities revolt.
In the same way, converting you country to a new religion could lead to the revolt of cities devoted of the former religion.
This revolt would spawn a new civilization automatically at war with you.
2) THE REBELLION OPTION:
Obviously that rule is hard that no one would like to change civics or convert their civ to other religion.
A better idea would be that the revolts could lead to divide your civ in 2 factions: "loyal" and "rebel". Loyals and rebels are allways at war, but foreign countries can´t attack rebel cities if there are not at war with you. Civil war would only happen if your country is at peace with everybody, and if other civ declare war to you during civil war it ceases and both loyal and rebels re-unifies to combar their common foes.
In exchange, when a civ is under civil war status, other civs would have a new diplomatic option: "support rebellion". That would mean that they are at war with the loyals and allied (free passage) with the revolts, and in case of capturing a city to the loyals, the city would be automatically owned by the revolts.
This way The participation in other country civil war would lead to no territorial gain. The benefits of giving support to the winner side would be just a better relationship with the country after the civil war´s end.
There could be also a "support loyal" option.
A peace teatry beetween loyals and rebels could lead to the creation of a new civilization with the rebel cities.
Example: AMERICAN HISTORY X. The english civ in the XVIII cent. has expanded and has cities in other continents.
After changing civics to "absolute monarchy", of after increasing taxation, some of the far cities revolt forming the "republican side".
The republican faction, after controling all the far cities, with a little help from the frenchs, offers peace, if the loyals accept that could lead to the creation of a new civ: "the americans", that would start at the XVIII Cent. as a republic and not at 4000 BC as despotism.
The new country would have the same civics of the older one, except that which has been their "cause" (republic in this case).
Later in time the american player decide to ban slavery. This causes a rebellion of a group of cities and a civil war.
By defeating the rebellion taking all the rebel cities the americans elude the creation of the CSA civilization.
Sorry for the lenght!