Well, it wouldnt technically be a "play in", the one game do or die would be a full fledged postseason game, count for postseason stats, etc. They would offcially be postseason births.
This is just one of the scenerios floating around, because it "punishes" the WC teams, gives a huge advantage to the #1 seed (in terms of pitching rotations/rest), and doesnt push the World Series back a single day. I love it, and hope that's what they ultimately do.
There is also talk of going back to the old 154 game season, or 156.