MC is not the emperor yet though. He's still just a regent with a strenuous hold on actual power.
If he manages to return from the west with the allegiance of the western kingdoms / democracies his hold on power will be much stronger.
But even if he were to be crowned King he can't really go around marrying anyone just yet, because the family of his wife wouldn't really like it. A nobleman marriage is a game of alligiances after all, and the north / south are lands you're looking to annect anyway.
I believe it will happen, but much later in the story.
I promised Cass I will marry her and hellbent on keeping my promise.