Megan (Jessica Paré)
Obviously, the largest unresolved plot point from last season* is Don's spontaneous proposal to his secretary Megan, who seems to have won Don over with her gentle, nurturing presence on an emotional trip to California. General consensus amongst fans of the show seems to be that the suddenness of the proposal means that it's a mistake for Don, and that the show will begin the new season with that situation already resolved. On the other hand, I think Matthew Weiner might be
working through some of his own issues with this plot, which would suggest that the next step is for Don to promote Megan to Vice President at Sterling Cooper Draper Pryce, then have her win a Clio, and then get fired. And then Don goes bald. You heard it here first.
*(other than whether Glen is being set up as a future school shooter or if Matthew Weiner's son is just really bad at acting)