*Only read the OP*
I prefer another approach to the problem you point out. I call this problem the "longevity curse"
and yes, is one of the most annoying things in the game (personally I can't think of any other more annoying). My approach is: let your character, as head of your dynasty, take decisions regarding your direct lineage. For example: your heir, the first son of your heir, the first son of the first son of your heir (these are three persons at most, I can't think any guy could live longer).
These decisions you can take are basically education and marriage. You still cannot decide wars, plots, ambitions, etc. for them, so you still manage one character each time. How you do this should be quite easy to implement. In the diplomatic screen when you decide marriages and education you can select anybody in your court, just add these characters like if they were in your court. This way you can follow an strategy for your dynasty. Of course, they could put modifiers... for example if your descendant has a negative opinion of you he could deny your wish.
But this thing is something Paradox should be implementing sooner than later, because it's plainly frustrating and breaks the charm of the game at some point. And also, you're telling me you, as king of Ireland want your recent baron son to marry some chick you decided it's better for your interests and he's telling you no?
P.S.: the abdicate button should be cool for some situations as in reality some rulers did it, though not very often. But it can't be the main way of strategizing your dynasty. It'd be cool if the character reaches some age and has some modifiers (like depressed), or reached some ambitions (paragon of virtue, this way he'd go to a monastery).
I hope Paradox reads this post because they should be aware it's a main problem for players. Come on Paradox, give us a solution.
