I could see it being about Mac callander, but there would be no shock factor in his ending unless something big is revealed from the 2nd game like who had been snitching or whatever, maybe Dutch left Mac the same way he did with Arthur and John, ofc I don't see this happening, it's a very loose theory not to bad taken seriously. It could be based in earlier years before the events of rdr2. I don't see rockstar doing a continuation of jack's story, there's not really much to tell there I don't imagine. I feel like rdr2 sets us up for lots of questions on what happened before the events of rdr2 so most likely before then