Neither. Before rdr2 sadie was a rancher. After she goes to south america. Which kinda breaks the whole wild american vibe
As for jack, I don't think they could make a story with him that'd interest me. Maybe playing as young hosea/dutch but even that's a long shot
And you guys forget that rdr2 mad a masterpiece with some new character that was very relevant to john but had no mention in rdr1. They could do the same again. Or we could play as mac or davey, someone we heard of but never really get to see.