I was thinking (always a risky proposition): maybe the immortality trait could be made as something that needs to be purchased during character creation? That way people who don't want immortality can spend the points elsewhere, while those who do can purchase it at the outset. To each his own while keeping it within the character creation process.
Another possibility would be to make immortality a high level spell. This also has some merit as it would make immortality a choice as well as a magical goal (you could even make an entire quest out of it!).
As for the AI, perhaps just a option during game creation ("AI's are immortal at start: yes/no").
Yes, this would be perfect Rooks. In 'The Quest for Immortality', you could explore aspects of your ruler's personality, as Arwen had to question relinquishing her immortality to marry Aragorn. Perhaps he/she falls in love with an immortal man/woman, and must choose the short road or the long one, with all the implications of each. Sure, immortality would be quite fun, but imagine all the nappies you'd have to change! You could even go the other way, with your ruler beginning as immortal, but choosing mortality for the sake of his/her mortal partner. Bring back the romance, I say..
In general, I think it would be cool to implement an even more aggressive dynasty system so that not only does the sov have kids but npcs can marry and have families too. Recently I played a game where my sov had to basically kill half his grandchildren because of a war a former friendly state. This kind of brutality which should cause rifts in your dynasty actually did squat.
This would be great, I agree. Or at least, your NPCs should be mortal too - or it would seem a bit weird with your royal family coming and going, and the other faces of court remaining eternal. Whether they do marry and have kids could be random, to cut down on the number of sprogs running around causing mayhem.
Eh, this [successor system] has been suggested many months ago, but since it's against the game lore [Sov dies = game over], some decent coder will have to mod in this. That would be a very popular mod I guess.
Yes, I realise that this idea must have been discussed many times previously. I'm sure that many people would love to see it implemented, in one way or another.