That seems like the way to go. Some people may want an entirely separate story and characters like GTA but I think these games being connected makes them more interesting.
You can't really go forward in time because the Cowboy era was already nearing its end by the other games so you can only go backwards.
This game mentioned quite a bit of history without going into too much detail. Dutch was in a partnership with Colm. He met Hosea and formed the gang in the 1870's. He met the 14 year old Arthur and later the young child John and saved him.
"Between 1887 and 1899, the gang carried out roughly 37 different bank robberies in various locations around the country."
Dutch killed Colms brother with Colm then killing Annabelle, Dutch's lover. There were other members of Dutch's Gang who were mentioned and had died prior to this game like the two Callender brothers and Jenny Kirk.
So there's clearly room for a game to be had here. If RDR2 was about how Dutch's Gang fell apart then I think RDR3 could be about how it rose up in the first place. Have the game centered around Dutch from where he was partnered with Colm up to them losing their money in blackwater. Set more in the heyday of the Cowboy days of the previous games.
It wouldn't surprise me if that was meant to be the plan all along. Things like how they referred to Dutch killing a girl in a violent way a few times but never showed it or much came of it afterwards...but leaves it open to actually show what happened in another game.
And again like RDR2, the next game could bring back the previous games location while adding in a whole new location with it.