With Red Dead Redemption II released, the game has so far been a huge success, though I can't say the same for Red Dead Online, and has garnered the wiki much attention. That attention has continued and looks to continue for some time. Though a third RDR game hasn't been confirmed, and would likely take years from now to be released, that gives us plenty of time to speculate on the possibility of a third game.
Red Dead Redemption III: Jack Is Back
The idea that I like the most is the idea of a Jack Marston outlaw story. Following the murder of Edgar Ross in 1911, Jack's like becomes a mystery. While some fans like to think he became involved in World War I, I think he either became an outlaw, or lived on the failed ranch in Beecher's Hope and became an author. I wouldn't mind if Rockstar decided to confirm that he became an author, though the hope of seeing Jack as an outlaw would be crushed. If they do decide to go ahead with a Jack outlaw story, I think it could go like this.
The game begins in a cutscene without any sound other than music showing the events of the Remember My Family mission. Jack's back is seen as he walks away from the graves of his parents and Uncle, then he's shown from behind while talking to the Bureau agent in Blackwater. It cuts to him riding through Grenehollow and arriving at Lake Don Julio, where he meets Ross' wife. After a few seconds, it cuts to Jack passing Fort Mercer and crossing the bridge into Mexico, where he meets Ross' brother. It then shows Jack approaching Ross, with Ross turning around saying "Do I know you?". The screen fades to black, with Jack heard saying "My name is Jack Marston", then a gunshot is heard. Footsteps are heard, and Jack is finally seen as he's walking back up the hill. In the original game, the game's title was shown at this point. In the new game, the new title would be shown. The screen would go black, and show a caption saying "A few months later". Jack is shown somewhere in the woods of Ambarino, the desert of New Austin, or the mountains in upper Big Valley.
At this point, I can't possibility predict where it would go from there. Perhaps I could do that in a series of blogs, a fantasy series which would only serve the purpose of entertaining the reader. Jack could form a small gang, though that's doubtful, considering I believe he may have wanted to be on his own.
Even though the old west would mostly be dead at this point, except for perhaps New Austin and maybe Nuevo Pariso, it would still be something I'd like to see, simply for the story.
Red Dead Remastered
The headline is self-explanatory. It's just Red Dead Redemption, but remastered with better graphics and the new areas of New Hanover, Big Valley, Ambarino, and Lemoyne. It would be interesting simply to see how those places would look in 1911 and 1914, plus it could lead to a new questline for Jack, giving us something to do in these areas. I would say that perhaps this should include the Jack outlaw story, though including the story of Red Dead Redemption and a new Jack storyline might be to much for one game to contain. So, maybe it could be a way to confirm Jack becomes an author living at Beecher's Hope.
Red Dead Redemption III: The Con Men
This one would involve the forming of the Van Der Linde Gang, with Dutch and Hosea. I don't really see much point in this one, since we know some things about the forming of the gang. However, it seems to be an idea people like, so maybe there's potential in it.
Red Dead Revolution, Red Dead Rebel, etc.
After Red Dead Redemption was released, most people assumed the next game would be a different story set in a different timeline, with a different meaning for the RDR acronym. This was the case after Red Dead Revolver, but would not be the case for Redemption. With the announcement of Red Dead Redemption II, it was effectively confirmed that it would be a prequel. Now, with the story of Arthur Morgan and John Marston told, it seems likely that if a third game is made, the story will either be about Jack Marston, or a new story involving someone else. Whether or not it would take place in the same canon as Redemption is hard to determine, though I believe it will be in it's own canon. This is probably the most likely option Rockstar would take, as it allows them to use a lot of creativity and make an entirely no story, with no boundaries.