I wouldn't say heir production is all that simple.
Some rulers are monogamous. Others aren't. Some don't marry at all, and just adopt an heir.
With monogamous rulers, there are generally kids, but not always. The eldest son may inherit, or the king may choose his favorite. Daughters may or may not be permitted to inherit. If the king has no children eligible to inherit, a nephew may be chosen instead. There may be disagreements over the rules of inheritance. In a society where the eldest son has always been chosen, there may be conflict if a kings wants to appoint a different son as his heir. In a society where the status of women is controversial and the king has no sons, some people may support the daughter inheriting, and others would prefer to see the nephew. Some people may take sides in these conflicts not out of idealistic attachment to a rule of inheritance, but based on who they would rather see on the throne for personal or political reasons.
With polygamous male rulers, "lack of an heir" isn't really much of an issue. If one wife is incapable or has a difficulty bearing children, there are plenty more. If the male ruler is even slightly fertile, he will at least have some children. So it becomes more of a problem of too many heirs. Take for example the Ottoman empire. Throughout much of their history, the ascending Sultan practiced fratricide to eliminate dispute over who the appropriate heir was. As soon as he had his first healthy son, his brothers and their sons would be executed, to prevent civil war. At some point, they stopped doing this, and simply locked up their brothers instead. As a result, when the Sultan died and was succeeded by his bother who had spent most of his life in prison, the new Sultan was insane, which lead to decline.
Some of the best Roman emperors were adopted. I guess they didn't suffer from the flaw of having been spoiled in their youth (or driven insane like the people who went from prisoner to Sultan). Plus, imagine actually being able to pick a qualified heir, rather than being stuck with the luck of the draw.
I'm not sure how to best represent this realism without throwing gameplay too out-of-whack. A few random events would add depth, but nothing too drastic. There will always be an heir. There might be some bloodshed before one is chosen, but someone will win. It will usually be a member of the civilization. Merging of civilizations through marriage can occur, on rare occasions, but even if rules of succession gave a foreigner a legitimate right to the throne, being ruled by a foreigner is generally not preferred by the people, who would fight to have one of their own inherit. Other times, a civilization will split rather than decide on just one heir, but again, this is rare, since they will generally just fight each other until a winner emerges.
And by fighting, I don't just mean armies, but also popularity contests, assassins, etc.
Internal politics events could enhance gameplay, but I wouldn't want to get too bogged down in them to the distraction of all else.
