This would be one of the very best features possible. In fact, it's almost odd this hasn't been mentioned in earnest before - I don't think I'm alone in saying that as soon as I read it, I thought "Of course! This would be amazing, of course we want this!"
It'd be extremely interesting, particularly if we saw a Romeo and Juliet-style occurrence - it would add an extra element to the "two courtiers got married without permission" event, and add a lot of additional behaviour that would fit so well in the game world. Plus, it also removes the ability for you to 'game' the world - currently it's too easy to, when you're old, simply go tyrant and revoke titles left and right, crushing rebellions, knowing that when you die your heir won't have to worry about the tyranny, and thus get a really well-designed realm. It's too easy to do this right now, and while I always think that personal relations should take precedence, dynasty relations should have a significant impact.
I'm going to throw my voice behind this one too - please, Paradox, make this happen!