As of the moment there seems to be very little risk of your character dying without a bunch of children - marry at 16 and you are all but guaranteed to have a brood of five or more princes and princesses running around your palace. My suggestion is that there should be a hidden trait, 'Infertile', which affects around one in fifty individuals (I think that is the rough prevalence), and would make it impossible for that character to have children. This would in turn lead to more interesting successions with different branches of the family inheriting things, and would make the game more unpredictable in general, which is always good.