So here are some suggestions I have based upon an old PBM system.

Whenever a Sovereign dies, there should be a chance for a Civil War, even when there is an obvious heir who will take the Sovereign's place as ruler of the country, empire, etc.

Chances of a Dynastic Failure

Base chance of a Dynastic Failure is 10%.

Modifiers (a few examples)

Monarchy +10%
Dictatorship +10%
Imperial +5%
Theocracy +5%
Federal Republic -5%
Caste Society -5%
Tribal Govt +5%
New King is not of age +10%

Specific techs could reduces this by -10% (Civic Techs?) (in the other game it was Bureaucratic Levels)

Each faction in Elemental could have modifiers.

The charisma of the Sovereign's successor could be compared to the other leaders could be factored in.

Leader Rebellions

Each leader when created would have a hidden loyalty rating.  During a dynastic failure there would be loyalty checks for all leaders.  Leaders who failed their loyalty checks could form successor states.  Troops commanded by that leader would then revolt as well.  If in the same region there were more than one leader that revolted and one or more that did not, the troops were split.  Revolting leaders without armies outside of the nation were eliminated.

Successor State Formation

There was a elaborate complex system of checks and rolls that would be made here.  Basically it determined which leaders would go to the loyal faction and who would go to the rebel faction.  In cases were they were in the same region, conflict was resolved to see who gained control of the region.

To make this relevant to Elemental, you could roll to see who's loyal to the successor.  The others could fraction off into their own nations.  If a rebellious leader is located within a city (or maybe nearby with a larger army) then the city could flip to the rebellious leader. 

Other Causes of Dynastic Failure

So there were other times when a Dynastic Failure check was made besides the death of the ruler.  Now I know that rebellions weren't in the rules for this game, but if they were here are some cases when a check could be made.

• Conquest of one's capital.
• Defeat of the entire national army by a foreign power.  For Elemental perhaps if the Sovereign's army is defeated there could be a DF check.
• Strongman's overthrow - when a leader has at least two times the troop points of the ruler, and the leader's combat & diplomacy (in Elemental's case it could be Charisma) were greater than the ruler's ratings then there was a loyalty check made.  If it was failed then there was a chance of a Dynastic Failure.  Maybe a bit harsh for this game, but you could raise it to maybe 3-4 times the combat rating.  Add in faction %'s or something.
• Major espionage actions performed against the country.  (Elemental doesn't have Espionage in it though)