What would have happened if instead  of getting divorced, Henry and Maddie had another child, a girl.  How different would Shawn, Henry, and Maddie's lives have turned out? Would they have still gotten a divorce after the baby was born? Would Henry have treated Shawn the same way if there had been a sibling? Would Shawn still have run away? Would Henry have let Shawn back into his life? Would Henry's attitude about things be different 

Trying to think of ways of to write this. Someone can do a serious fic, where Shawn finds his mother's old positive pregnancy test, then dreams about the way his life would have been different. 

Could be written as a parody of Buffy, where a little sister suddenly appears, and everyone just acts like she was always supposed to be there. But turns out she is some type of inter dimensional key who was created so Shawn would protect her like a little sister. 

 This challenge was inspired after reading a story where in order to write a crossover, the author randomly gave Shawn a younger sister. There was no explanation given as to how differently Shawn's life would have turned out if there had been a younger sister.