Just to explain, I'm doing a game started with a count in the ERE, and his dynasty became the principal dynasty of the empire (except that I didn't overthrew the emperor... yet). I used the CK2+ mod for the patch just before SoI (because I wasn't to scrap my game and apparently the muslims are just too strong in this version). I was in the early 1300 when my ruler died after more than 50 years of good rule, having inherited the throne of the Nicean kingdom at the age of two. The only problem was that he didn't have a male heir. So his daughter inherited. BUT, just a little more than one month later, the previous wife of my king gave birth to a boy!
However, the lady reigns now since more than 20 years and never once did she had been overthrew in favour of her little half-brother which is a duke. Her heir is her kid and the little brother is in third place. Isn't it sad that I didn't had any kind of rebellion for this reason? This guy would have, had I been in SoI, something like a giga-claim on my whole despotate, but nobody seems to have seen this. The emperor could, had he been wise and the game had given him the opporunity to do so, have destroyed my realm unity which is now more than half of the empire and avoided the very possible outcome that I will at somepoint overthrow his dynasty. I would add that, had these option existed, I would probably have done everything in my power to suppress the threath, but now I had no incentive.
So, I just want a mechanic to represent the greater legitimacy of this son than my current ruler's and the oddity that is her reign.