(Suggestion)Sibling creation, rivalry and ideas.
I was recently playing a game and was thinking to myself offspring are cool, but is that it?
There has to be a reason to make them more important, make the game deeper etc.
I was thinking of few ideas to make it work better:
1) The offspring take on a combination of both parents stats/traits etc. Plus maybe a random trait?
2) At the moment it tells us that the first born are first in line...but they never get there or really have any effect. Lets make their stats useful in some way such as adding to the morale/traits/kingdom.
3) I thought perhaps an aging system but then you may have your SOV die...which means late in the game you could have a very weak leader. Perhaps, the SOV should be immortal but the kids age and die.
4) Give them the ability to rebel and leave if something triggers this...they leave and start their own nation, join another faction etc.
There has to be a bigger emphasis on diplomacy then we have now to control some of these ideas (It is to easy to whipe out a faction with no negative reactions.)
These are just some ideas, feel free to comment or give your own.