For single player, speculation is that it's going to be a prequel, since John Marston died at the end of Redemption and to bring him back would mean to go back to when Martson was part of part of Dutch's Gang. This would work perfect if they want to do character switching like GTA5, they could make it so you switch between Marston, Bill Williamson and Dutch van der Linde. It's not necessary to have three main characters like GTA5, but Rockstar said they really liked how that turned out and I think would work well in this game too if they go back to when Martson was in a posse.
Back to online, really they just need to add RPG-style customisation and online money and the rest will fall in place.