A thought in the subject:
When a sovereign dies, instead of "game over" the kingdom will split between the children (net necessarily an equal division), allowing the player to choose which child he will now control while the rest become NPC kingdoms.
With force or diplomacy, the PC can reunite the kingdom, but it will demand time and effort.
This will allow the game to continue after the main hero dies, but with a big penalty.
To prevent an abuse of wanting only one child for a "smooth" succession, some cities can become neutral with no leader or even completely destroyed when the sovereign dies.