Although not an outright inheritance as suggested, I wish there was an element of war of succession for a faction/province/state? Highly unlikely in this game, but what if no clear male heir is available, then factions with princesses from the disputed faction can lay claim to it and be a mission? For ex, King of Engld passed away and no male heirs could be found, since you are married to one of his daughters you can claim his lands, advance to london in x turns with an army and defeat all other clamaints already there, or something to that effect. How's that?