I agree with you that they almost definitely WON'T go farther into the future, but I disagree entirely that it wouldn't be a western if they did. The genre conventions are flexible, and they're usually more interested when they're stretched a bit. Look at Yojiro or No Country For Old Men. Both stretched the definition but justified themselves entirely as westerns. The neat thing about "western" is that it really is a proper genre and you can set it in almost any time and setting if the writers/directors/developers are talented and careful enough. You could have a western in 90s LA framed around gang rivalries. The film Black '83 isn't exactly a western but draws heavily from the genre by using Ireland during the famine as the lawless, looming frontier and the British occupation vs an Irish rebel takes on the place of American occupation vs heroic Indians in the anti-western strain. There are also movies set in the classic western era with similar characters and setpieces that couldn't really be described as drawing from western genre conventions. The Revenant is a great film but is more of a mature adventure revenge epic. The new Lone Ranger movie is obviously shit, and there's plenty of shitty westerns, but it didn't even try to be anything other than a family friendly action Blockbuster. The Legend of Zorro is better and more fun but is far more of a swashbuckling Robin Hood/Three Musketeers than a western despite the setting and time.
All that to say that Red Dead could go in a lot of directions. Of course they should keep within their own thematic and narrative precedents, but they can and should stretch them pretty far to make it interesting. RDR2 is so good because, despite the enormous sales success of GTAV, they didn't settle on only making an updated version of RDR1, which was basically GTA: Wild West with overwrought caricatures and cheesy humor. Instead they made it a mature narrative and took a big chance by making the basic gameplay mechanics – everything takes much longer and feels more measured, fast travel isnt easy, youre encouraged to camp and hunt – themselves reflective of the genre and narrative.
I don't have a strong preference for where they'd go with the next game, but I'd prefer if they took a chance on the setting and moved away from the southwest/rocky mountains. The Pacific Northwest would be very cool. You could do it during the Oregon Trail, a recently underused western setpiece. You have a variety of environments, both recognizable and new, from prairie and desert to temperate rainforest and chapparal california coastline. Instead of being about the death of the west and that kind of disillusionment, it could be about the decline of native nations as over the course of the game you see the dynamic change from still-independent and thriving native societies and a few american outposts give way to proper american towns and ranches and wars with sidelined native societies. I dont expect, and almost wouldnt want, Rockstar to do this because it would mean dealing with native genocide and I dont think they could handle that properly walks the line between being an honest portrayal and being non-didactic.
But you see what I'm getting at. I also really like the other anons idea about having it focused around Jack in prohibition era Mexico. You could very easily set a western in contemporary Mexico, and Mexico very much was still mostly peasantry and using horses well into the mid-20th century, so it's not like it would be flapper girls and tommy guns or anything like that. Wide variety of environments, Mexico was still in the midst of revolutions and general social upheaval, booze smuggling in ports and border towns would allow for a wide variety of characters from all different backgrounds.