Since everyone is so adamant that no one will guess the cliffhanger, I've come up with some crazy theories of my own. Prepare to lolz.
Shawn inherits a ক্যান্ডি চকোলেট factory and moves to Poland
A shape-shifting soldier from a parallel universe kills Gus and takes over his body (cue Fringe crossover)
Shawn and Lassiter find out they are related
Shawn and Juliet find out they are related *dies*
Santa Barbara is overtaken in a zombie apocalypse
The chief is pregnant…with Lassiter
Yang enters a radio sweepstakes to win two tickets to a monster truck rally and invites Shawn
Shawn finds out he’s adopted
Mary wins the racquetball world cup
Lassiter is Yin
Shawn’s mom is Yin!
Shawn finds various missing toys from his childhood in Yin’s apartment
Lassiter leaves the police force to catch his big break on American Idol
Shawn finds out he’s just been hijinxed দ্বারা Henry (at least one of আপনি must have seen that প্রদর্শনী on Nick@Nite)
Mr. Yin is actually a toy poodle named Scruffy
Yang has been remote controlled দ্বারা a man behind a curtain all along
Shawn gets hand-picked দ্বারা CTU and Jack Bauer to help stop an assassination against the president
Shawn is forced to face his fear when Patrick Dempsey turns out to be Yin
Juliet receives a Hogwarts letter
Yin turns out to be none other than Alfred Hitchcock, himself
Yang gets chosen to be a contestant on Survivor: Antarctica
Lassiter decides to embrace his childhood দ্বারা bidding on a Razor Scooter on EBay
Enjoy Mr. Yin! 13 days!