Okay, first of all this has not been confirmed and is just an idea. There are spoliers in here.
Jack meets the stranger from the I Know You quest, who tells Jack Archer Fordham's wife has been sighted near Tall Trees, he tells Jack that he may get information from her, using violence or doing tasks for her on where Archer is for revenge on killing his father. Jack tracks Archer's wife down, where he either threatens to kill her or helps her by picking flowers. She tells him Archer is located deep into Mexico, but that is all she knows.
Jack travels to Mexico, where a gang he meets captures a small Native American village to rape their women. Jack tries to stop them and kills most of them before being shot in the leg and knocked out, his horse already dead. The gang leader is standing over Jack, ready to kill him, before he is shot in the head. The stranger from before wakes up Jack and tells him they need to move, or they'll both be killed from reinforcements. They move down (where before was not assessable from in game limits) They succeed in escaping, but Jack is knocked out and the stranger disappears.
Jack wanders into a small town where he finds Archer. Jack attempts to kill him, but while aiming for Archer, and everyone playing poker, gets up and draws their gun and shoots Jack where he is again knocked down, and passes out. The player takes control of John, busting though his barn door, Jack hallucinating, and John fires, but this time manages to kill everyone. Jack goes back to when he was sixteen, as a wave of Army soldiers ruthlessly kills Jack and Abigail, showing what wold have happened if John didn't sacrifice himself.
Jack wakes up in an Native American village, where the natives saved him from the government, and the Chief claims they will help him kill Archer in any way possible. Jack and his new gang of young Indian warriors go back to where Archer was at before, but this time they're ambushed by Dutch's Gang, whose superior firepower (Winchesters against bows and arrows) ultimately wins and Jack is separated from his gang and takes cover, and ultimately decides to make a camp and go to sleep
Waking up, Jack decides to decides to go search for his gang. He finds the town has been turned into a gang hideout. Walking through it, he can women scream, and finds several naked bodies of dead young women and several dead and scalped corpses of men. Jack finds the gang raping women, until he starts shooting. Jack clears out the hideout, and finds his gang, only four of the ten warriors alive, and each one severely beaten.
They scavenge weapons and ammo from the camp, and decide to go kill Archer, as he has been spotted in Chuparosa. They go there, but it was a trap, and one of the member of Jack's Gang is a traitor. The traitor whips out a revolver to kill Jack but misses due to his inexperience with firearms. Jack challenges him to a duel, but he is hit as the traitor duel wields two revolvers, but Jack wins. There, he gets the Twin Revolvers to use.
Entering town, the stranger shows up and tells Jack and his gang that Archer left this morning for Blackwater. However several Army troops and a General, each one responsible for the Marston's Ranch Raid are stationed there, ready to ambush them, but tells him the General has a family, so it's his choice. No matter what, Jack and his gang raid the town and kill all the troops stationed there and capture the General. Jack has the option of killing the General or letting him live, either way they travel to Blackwater.
While looking for Archer, Jack and his gang are ambushed and Jack is forced to play Archer in a game of Liar's Dice, Black Jack, or Poker, and for every hand he loses, a member of his gang will be killed. The game will keep going until Jack loses a hand, Archer will always buy himself more chips. However, when the first member is killed, Archer no longer buys new chips, if Jack loses another hand, the mission is failed.
Jack eventually wins, where Archer says in order to duel him, he must win a shootout with the police. Snipers are set up in case the gang tries to do anything smart, but no matter the challenge Jack's Gang will win, and will clear out the snipers. Archer attempts to escape, but his horse is shot and the gang walks over to Archer, ready to kill him. However, while putting his revolver to Archer's head, Jack's Gang betrays him, pointing rifles at his head. Jack simply draws his Twin Revolvers and kills them.
Archer takes this time to shoot Jack three times in the chest with his revolver. Jack gasps for air as he falls, vomiting up blood and starting to choke. In his last moments, he draws a pistol, as Archer turns around, and fires several shots into his head, killing Archer once and for all, before dying himself. In the distance, the stranger and John Marston use binoculars to watch Jack's final moments.
John asks "What's gonna happen to him?" and the stranger replies, "His life is going to flash before his eyes, much like yours did." After this, the player is a given the option to see John or Jack's life, and if they pick John, they start right before starting The Last Enemy That Shall Be Destroyed or if they pick Jack and if they pick John they start from the last save before Jack meets the stranger.