I've seen many prequel ideas where you would play as John while he was in Dutch's gang. My ideas take place during the same time but would have a different main character.
The first takes place in 1899 and would be centered around Landon Ricketts. Parts of this have already been mentioned in Red Dead Redemption such as the Blackwater Massacre and the death of Bonnie Macfarlane's brother.
The second idea would take place in 1906. This could also use a character that already exists or introduce a new one. It would also mean that it could be you that shoots John in the boat robbery.
Both of these would give more information on John, Dutch's gang and the rest of the characters in Redemption but it would not have the predictable storyline of John robbing banks, getting shot and beacoming a rancher.