I'm sorry to see Dan Stevens leave DA and the demise of Matthew Crawley as a character; however, I think this could be a good thing for season 4 since there are all sorts of directions that JF could take. Happy couples just don't seem to intrigue viewers. As much as I loved the character of Matthew and found him to be one of the few honorable people on the show, what was JF going to do with him? I think viewers could only handle so much conflict between Matthew and Mary, Matthew and Robert, etc.

I could see any of the following happening for for Mary:

Mary takes on a more direct role in the running of DA as a way of carrying on Matthew's vision and protecting the future of DA for her son and DA heir. It would also coincide with the growing and changing roles of women in the 1920's.

Mary and Branson form a bond based on loss (and the running of DA) that may start to lean them toward becoming involved; however, they form more of a brother-sister attachment and an alliance to better DA and provide for the future of their children and keep one another safe and happy by helping each other avoid relationships with others that are not in their best interests

An assortment of suitors wanting to be close to Mary and possible step-father to the heir since they might be able to adopt a lifestyle they can't afford without the direct responsibility for DA. Think of how many poor and/or landless peers there were at that time . . . kind of like young versions of Lord Hepworth . . . or the return of the Duke of Crowborough (who would definitely get put in his place).