Way back in 2010, the gaming industry looked a lot different than it does today. Microtransactions had barely even reared their ugly heads and bigger paid DLC packs were the in-thing at the time. Major AAA titles often saw post-release content support in the forms of singleplayer story expansions which either added to the campaign or continued the story with further missions and chapters.
In the case of Red Dead Redemption, we saw a rarer version of the DLC manifest - the standalone expansion. Standalone add-ons never were common in gaming, but even less so these past years. Nonetheless, Red Dead Redemption recieved a standalone content expansion in the form of a non-canon zombie game set in the same location as the main game, and with the main characters. Rockstar opted to develop Undead Nightmare as they've "always been interested in making a zombie game", but instead of kicking up a whole new IP, they just tacked on the theme to an existing one.
However, since Red Dead Redemption is grounded in reality, Rockstar opted to go non-canon and standalone to make sure they differentiate the two from each other well enough. But could Red Dead Redemption 2 ever see a similar expansion? Maybe instead of zombies, maybe there would be an alien invasion, or demons attacking from a gate to hell.
However, looking at the state GTA 5 is in, chances are Red Dead Redemption will never see single player expansions. GTA 5 hasn't seen any story DLC in the three years since release, and the success of GTA Online has pretty much guaranteed that it will never see such an expansion. We already know RDR2 is going to get a similar multiplayer mode called Red Dead Online, and that it will get frequent DLC, meaning there is a very good chance that Rockstar will pursue the same update model - lack of single player included.
Would you like to see Red Dead Redemption 2 get a singleplayer expansion?