Similar to the happy ending, although it doesnt necessarily have to have that happy part. It could take things 10 or 20 years into the future. Peggy could have fully completed her transition into Don Draper II (or should that be III?), heavily drinking and enjoying a string of cheap one-night stands. Roger would probably be dead by this point. The most interesting person to follow after such a time period would be Sally Draper, because there is no telling what those emotional scars are going to do to a person, yet you could also see her absolutely end up being President.