Red Dead Redemption 2 is basically an instant classic for Rockstar Games, meaning a new numbered entry only seems like a matter of when, not if. Since RDR2 is a prequel to the original, it remains to be seen how the next game manifests. It could just be some sort of interquel between the two games, although big events there are mostly filled, or perhaps more likely it is a direct sequel to the original, putting players in the boots of Jack Marston.
Given Jack's role in both games, assuming he appears in the next game is a given; however, it doesn't seem like he would be the only. Both games have featured two protagonists, with one taking lead and the other following up. In RDR1, that's John Marston followed up by his son; in Red Dead Redemption 2, that's Arthur Morgan followed by John Marston. Putting them in chronological order means the secondary has, at least in the case of John Marston, followed the primary in the game.
In other words, it would make sense that Jack Marston was the primary protagonist of a direct RDR sequel, but the secondary character is left in the air. There's one perfect character who could fulfill that role, however: Sadie Adler.
What Happened to Sadie Adler After RDR2?
In the RDR timeline, the last time Sadie Adler appears is at John and Abigail's wedding. It is assumed she left for South America after that, joining Charles Smith in the couple of RDR2 characters to survive and grow better from their time with the Dutch Van Der Linde Gang. She's never mentioned in the original game, so for the most part, her story is a blank slate in this regard. This means that she could return in RDR3 at Jack's side. It is worth mentioning that she and Jack were never particularly close, but she was close with Arthur and John. That relationship and in honor of his memory would be compelling reason enough for the two to cross their paths.
Furthermore, Sadie Adler is the perfect age to step into shoes befitting that of Arthur Morgan or John Marston. While her exact age is never revealed, she was married to Jake Adler in September 1896. Assuming she's in her late teens, this means she's early-to-mid 30s, which also suggest she would be in her mid 40s come 1917, the end of RDR1.
Sadie Adler in Red Dead Redemption 3
How this would be implemented is another question, but an easily answerable one. Perhaps she comes back to the United States to visit John Marston to instead find Jack, perhaps she has heard of his crimes as a Bounty Hunter and is chasing him, or perhaps she is seeking him out after hearing of John's death. The game would have to recognize the changing times and how WWI was beginning around this time, but there's a lot that could be done. As aforementioned, Sadie is a blank slate for many years, allowing the developer to explore her character easily.
Even if it took place in another time, it would be interesting to see how escaping Dutch's gang changed her as a character, and it would be a real twist of fate for her to develop some type of maternal relationship with Jack. Presumably, Jack finds himself after the deaths of his loved ones in a genuine "Last of the Outlaws" position, and Sadie may actually have a legitimate career as a Bounty Hunter now. Either way, it seems like a real recipe for Red Dead Redemption 3, whenever it does indeed release.
Red Dead Redemption 2 is out now for PC, PS4, Stadia, and Xbox One.