First and foremost: A side campaign like they did with GTA4's 'The Lost and Damned' and 'The Ballad of Gay Tony', with players playing as retired circa 1914 Marshall Leigh Johnson to showcase either his life during John Marston's campaign against Bill Williamson, his actions after Marston's departure from the country, or after his retirement, I believe it would open up the possibility to dual-wielded revolvers (as a reference to Red Harlow's ability to in Red Dead Revolver, considering he holsters two Cattleman Revolvers)
Shoe-polishing in Blackwater, your boots get trail dust from being in the dirt, there is a booth adjacent to the train depot, the price being $3.
The ability to rob trains, maybe by making the conductor tell you the code to a safe in the back or simply by making him stop the train and robbing each person individually.
The ability to allow multiplayer characters the emotes that randomly happen before a gang match or free-for-all, like smoking, repairing, drinking and playing music on a violin - in free roam and on command, even if only at Camp.
More silent films at the theater, possibly with the ability to select which film first-hand before entering your seat.
The return of John Marston. A cheat-code similar to 'Jack Attack' that allows you to revert back to John. (Maybe around the time of Undead Nightmare DLC.)
More ways to spend vast amounts of cash, either by purchasing a water raft, or the repairing of the 'Serendipity", or expensive outfits and weapons.
Access to Cochise! "Little Joe" Cartwright's pinto horse in the TV show Bonanza.