Apocalypse World

Alternate Universe 2017 is an alternate timeline set in 2017, which could only come about had Mary Campbell refused to make a deal with Azazel, which ensured her boyfriend John Winchester remained dead and their future sons, Sam and Dean Winchester, were never born.

History
After John Winchester is killed by Prince of Hell Azazel, Mary refuses to make a deal to bring John back. Mary remains alone and a Campbell for the rest of her life, rendering her future, pre-destined sons Sam and Dean Winchester as non-existent.

Azazel likely chooses someone else as a Special Child and future vessel for the Archangel Lucifer. The Archangel escapes The Cage and brings about an Apocalypse, decimating the human race while Heaven and Hell resume their war.

By 2017, one of the surviving humans is Bobby Singer, who is equipped with angel-killing bullets. In this universe, a class of demons known as Tempter demons exist.

Season 12
While his mother Kelly Kline goes into labor, the Nephilim Jack opens a portal that leads to this alternate universe. His protector Castiel falls into it and meets Bobby Singer there. He manages to escape the show this world to Sam and Dean. When Lucifer arrives in hopes of claiming his son, Sam, Dean, Castiel and Crowley decide to use this portal as a means to lock Lucifer away.

Crowley chooses to sacrifice himself as part of ritual that will seal the rift. Unfortunately, Mary Winchester ends up trapped on the wrong side along with Lucifer. Their current fate is unknown.

Aftermath
The state of this world encourages Sam and Dean to believe that they are indeed heroes who have saved the world, preventing it from becoming what it did in that universe. The major downside is Mary's absence, as well as Castiel's death which resulted from a struggle with Lucifer.

Appearances

 * Season 12
 * All Along the Watchtower

Trivia

 * This universe is similar to the alternate universe of 2014, in which Sam said "yes" to Lucifer, allowing the Archangel to use his True Vessel to exterminate the human population.
 * The main difference is that no one was left behind in that alternate universe.