I don't see how they could match RDR II to be honest, unless they only focused on improving game play and missions and kept everything else the same. For me, RDR II was the complete and definitive Western/Frontier story taking into account thousands of elements from real life experiences, real-life stories, case studies, Western movies, comics, newspapers, shows, theatre show, music and sounds tracks. It had everything and I can't think of a single thing that they missed out on from that era to be honest. Also, if you were to do a prequel set in the 1860s and 70s, maybe the environments would be less populated and towns much smaller so it wouldn't be as fun I reckon.
To even try and match the world of RDR II and add more would be a significant challenge. Why not try and do something different? Maybe late 1800s peak-immigration NYC in the style of Gangs of New York?