I have read two separate theories
One states that Don will come back to NYC, and pitch McCann with the "I'd like to teach the world to sing, in perfect harmony, I'd like to buy the world a Coke, and keep it company" ad campaign that ran for like 20 years. (the ad campaign started in 1971, months after the time of the show currently)
OR
That he will get a plane ticket, with all his cash, and then jump out over wilderness like D.B. Cooper.
http://en.wikipedia.org/wiki/D._B._Cooper who did all this in November of 1971.