Selection of new ruler after death + Kingdom buffs + Crown buffs (Ruler overhaul)
Whenever your current leader dies you should get the option to manually select who the next leader will be. If you opt to not select then the game with decided like it does now. This selection should be a big deal for your kingdom however. Just like the start of the game, every current noble more-less needs to "agree" to the new leader or, well, leave.
Another addition to this is to give the leader certain passive buffs and debuffs to the entire kingdom. This is going to (likely) complicate your choice and make you really need to think of what's going to be best in the long run for the kingdom.
For a rough example, my old leader just died and of course I can choose any of my nobles to succeed them. One noble that 80-100% of the kingdom loves will allow me to choose him and lose no nobles. However. I really need to focus on mining and get that gold! That noble that everybody loves has a -20% debuff to mining for the entire kingdom. On the other hand, I have another noble that is +10% to mining but only 60% of the people like him and 20% have straight up said they will desert right away if he is chosen. Risk..... reward....
Finally, the Crown Buff. Any ruler will age at 2x the speed due to the burden of leadership. However, if he wears The Crown that provides a buff that makes him live 1.2x longer than normal. This allows the player to have influence over their leaders health. If you're "done" with your leaders buffs and want to risk a new noble then take the crown off them like the cruel deity you are. Note: Crown has no effect on anybody that isn't the ruler.
+1 for a choice with consequences