If we're talking about what would be cool maps, I'd either like Mexico again (I was real disappointed they didn't include it in RDR2), or a Civil War/Reconstruction-era game along the Mississippi.
I imagine a map laid out tall and thin, with most of the content (maybe all of it) on the west bank, so you basically transition from Louisiana (Lemoyne) up to Minnesota with its bayous, up into normal subtropical forest, to the mountains, all the way up to snow-covered forested areas. And it could be laid out with two major cities, one representing Chicago and the other New Orleans. Extended to the west, some room for the Great Plains. Historically, this area was a hotbed of outlaws coming from a massive army being disbanded and sent home to face occupation.
I'd also like an RDR that focuses on Indians, like something set in the turbulent world of New Mexico Territory (Navajos, Apaches, Comanches, Mexicans, and Mormons in a desert environment, but it's not all desert, New Mexico has forests and honest-to-god snow), or out in the upper West, like the Black Hills and Yellowstone, with the plains nomads.
And it'd be great if somebody bothered to make a game set in the Old East or revolutionary/colonial days, like when "Northwest" and "Southwest" meant places like Indiana and Alabama. People think that a game like that would suck because of the guns, but that's why it would revolve around melee combat: you take your shot, then close in and start fighting with knives, bayonets, tomahawks, swords, war clubs, or fighting it out with bows and arrows. It'd be way cooler than cover-shooting with guns.
I loved RDR and RDR2, but I'm tired after two of them of the elegiac "the West is fading away" crap with Gatling guns and Mausers everywhere. The opening of the West is more interesting than its closing.
I firmly believe when rdr3 happens it needs to take place during prohibition and be about smuggling and bootlegging , you can keep rdrs themes and mechanics but also get some of that gta flair with car driving
I assume you've got something like Canada-to-Chicago or Cuba-to-America in mind, but the Appalachian moonshining was a huge source of liquor for the domestic market, it'd get produced in the mountains and then shipped into big cities. ("Lawless" is a good movie with that setting.) The setting is something that it's hard to imagine the public taking an interest in, but I thought RDR2 captured the Southern mountain feeling stunningly well with Roanoke Ridge, and it'd be interesting to me if somebody made a game with an environment like that in a Model-T and tommy gun era. It could have a large city representing Atlantic City, a mafia-and-moonshiners sort of deal. You could go from horses and hunting to jazz cabarets and casinos in the same game.
