Family dynamics is a common theme. With Mary being pregnant, it affects the whole family. There are new relationships to navigate and old ones to redefine. For instance, how her father, the Earl, might see his role changing as he's about to become a grandfather.
A third plotline could be centered around the reactions of the rest of the family. The Dowager Countess might be both excited and full of advice, some of which might be a bit old - fashioned. Mary's sisters could be jealous or overly helpful. And the servants would surely be gossiping and speculating about the new addition to the family. This could create a web of different emotions and interactions within the Downton Abbey household.
Definitely fiction. Downton Abbey creates a fictional world with made-up characters and events, although it may draw inspiration from historical periods and social contexts.