Bah! You Kingly types have it easy. Playing Duchies is a little more challanging.
In my long term beta-marriage experiment, I am Duke of Provence, with sons as Duke of Dauphine and Duke of Savoie (don't ask why both of them are independent
), and we "de Provence" family have Duchies of Toulouse, Poitiu, and Flanders (all quite large) in our grasp (a few more marriages are required
). All the aforementioned are from our dynasty, it is just a question of who inherits who. Not to mention the King of Germany, but alas, the Franconia name has remained strong despite my best efforts... And the Normandie branch of my family has some promise, but they also retain their name, so England is safe from my grasp for the moment... 
In my long term beta-marriage experiment, I am Duke of Provence, with sons as Duke of Dauphine and Duke of Savoie (don't ask why both of them are independent