A Kingdom for a dynasty!
Especially with the game now starting in 1419, I believe it even more important to include a dynastic model, which has been called for in numerous threads.
Consider Habsburg! This dynasty managed to gain a world empire primarily through marriage! But think of also all the fuss about Austrian and Spanish succession sparking off to pan-European wars. Or the Tudors, Jacobites, Hanoverians in the United (or first not so united) Kingdom. Being a Scot myself, I'd love to see my James reign both England and Scotland in personal union (not a united country! mark that!) after that nasty heretic virgin Queen.
I should think that the rules governing dynasties should be very close to Blood Royale - a boardgame or Diplomacy Royale.
Paradox, you simply must check out those splendid rules:
http://w1.132.telia.com/~u13206915/ROYRULES.htm
You'll be kicking yourself why you didn't think of such ingenious bits yourself!
Some people have mentioned that there's a problem that not all countries had dynasties and you've got all those Muslim and Pagan (or whatever countries).
Well, I'd simply say that republics etc. should simply not be able to have a dynasty, succcession and thus state marriages. Further, Muslims would only be allowed to have state marrieages among themselves etc.
Just to remind anybody interested in games with dynasties. I am going to GM a game of Diplomacy Royale. If interested, check out my threads referring to this matter. Links and info can be found there and not least my mail adress for signing on.