It needs to go back to the past, new storyline, new characters (I actually still wish that's what RDR 2 would've done). I grew tired of the "twilight old west is dying" atmosphere in RDR 1, let alone 2.
I want a game in the earlier west, when settlers were still moving west, right around when the Civil War was ending, etc.
My absolutely "dream" for the next RDR game would have you playing as two characters (with the game both switching between them for story elements and other things) and to have more rpg elements and for the love of God more FREEDOM in how you tackle missions and player choice (I hated how strict RDR 2 is at this, if you don't do a mission EXACTLY to a T how they decide you should you fail, even if there's logically another way to do it). More rpg elements also for your character, with skilltrees and stats, however unlike traditional rpg's these are not tied to some kind of "experience" rather they are tied to both the storyline (as your character ages and time passes) on top of having skills that you actually learn from multiple ways, such as an npc that offers to teach it to you, story related quests, etc. So basically there'd be no "grinding" and leveling up or anything, you'd just naturally grow/learn more things as you progress the story and explore the world. (also you couldn't just explore and pick up EVERY skill, npc's that teach them would show up at different parts of the story/time int he game so you can't cheat lol).
The first main character would have you playing as Native American. It would start with like a coming of age style storyline for this character, with you learning about the tribe and the character going through a vision quest/rites of passage. Then one day you are on a hunt and when return see soldiers at your camp, they are cramming people together, and marching them out (the trail of tears). You hide and when it's over you go into your village to see your immediate family scattered around camp except for your sister who was taken. Along with a few others that managed to hide they start hiding in the local montains/woods to survive but one day the character decides he isn't going to simply hide, you're going to go off and track down the people that did this and get revenge.
The second main character would be a soldier during the Mexican war. It'd started with him near the end of the war, introducing you to his character and his gameplay style. Then after the war ends he returns home to his family on a little farm to build your life there. Shortly after you return you head into town to get some supplies and some gifts then when you return you find your house and things burning. Then you find your wife, barely alive, as she tells you what happened, a group of Native Americans came through and attacked it, taking the horses and other things, she dies in your arms. Then you find ithe rest of your family all dead. This is where this character sets off on his tale of revenge to find the band of Native Americans that killed his family.
Then around 20% into the game the storyline picks up with:
The Native American character tracks down one of the people who was there, he beats him up and gets him to talk about some other people, then he takes the revenge he wanted but in the process he is caught and arrested then thrown in jail.
The Soldier meanwhile tracked down a lead, however the lead, doesn't quite want to spill the beans on what he knows, so you have to take certain measures. During this the guy ends up getting the draw on you and you are forced to defend yourself but you're caught shortly after and thrown in jail. It just so happens to be the exact same jail that the other character is in.
Though they both share prejudices against one another they need each others help to break out, as they know that they'll be hanged for their crimes. This is where the characters are forced to join up and then the storyline basically starts off with them at odds with one another but surviving on the run together, as they both become wanted. They learn over the course of the game just how wrong they are to judge someone based on others actions, coming to terms with those views they have, and trying to help each other on their journey of revenge, which spans years and years as they grow older and become close friends. Even after the Native American finally finds his sister and they both help set her free the Native American decides not to leave with her but instead to stay behind with the soldier character to help him finish his quest.
One key point of the game is that each character plays differently and you are free to toggle between them when you wish (outside of specific quests/stories). The other character can be with you on quests (so you can easily swap between them while on the quest) as well as the "open world."
When I say plays different, I don't simply mean skillsets, but the npc's and other things change depending on what character your playing and the npc themselves.
For example, if you are in a town, you might find that the townspeople are more friendly and open to talking to the soldier character instead of the Native American. Whereas if you're talking to other Native Americans they might be the opposite, it wouldn't be as black and white as that obviously but you get the point. Depending on the character you are playing as the dialogue and even the quests will change depending on the viewpoint you're playing it from.
Whew, I didn't expect to type ALL of this out but once I started I couldn't stop.